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my 87 lx 5.0 has tail lights that don't work when the headlights are turned on, there are no pad fuses and I tried a different used headlight switch that was supposed to work perfectly, I should also mention when I turn on the lights the digital speedometer goes dark as well as the rest of the dash lights, any fresh ideas would be helpful.
thanks!

First I'll pop your dash cover off and get the cluster out and replace all the blown bulbs

Some really odd  can start happening when your instrumentation illumination home lighting is all blown up

Let's start with that first

Next up click on the 87 EV TM link below and start studying the taillight wiring diagrams

Are you good with electrical?
Do you know how to use a meter?
 We may need you to use a test light or a meter to check for voltage at certain wires when the switch is in certain positions

wiring of course isn't my strongest area but I do know how to operate a test light and meter. Ill check out the link, thanks

Is everything stock?
Do you have the Autolamp feature?
What is the battery voltage with engine off and engine running?

Everything is stock,  i haven't checked voltage but I will, it doesn't have auto headlamps, there's a couple of parts behind the dash I have to check, I also thought it might be something in the ignition switch I heard they can overheat

Can't see any connection to the ignition switch for this problem.
From the EVTM that jcassiity linked you to in his post, print pages 77, 91 and 103.

Page 77 is the headlight section of the headlight switch. You see "hot at all times" power to the B1 terminal of the switch.
With the switch in the "head" position, power goes through the multifunction switch to the headlights.
Note there is no internal electrical connection between the headlight section and the other sections of the switch.

Page 91 is the external lights section of the switch. Both front and rear lights.
"Hot at all times" power through fuse 4 to switch terminal B2. This is the same fuselinked power source that goes to terminal B1 for the headlights.
There is power on terminal R with the switch in "Park" or "Head". This power also goes to the "Dimmer" rheostat in the switch and to the cluster illumination relay left of the switch.
(sorry have to go. will finish this post later)

Page 103 instrument illumination, powered from the dimmer in the HL switch.
Notice the cluster illumination relay. With the HL switch off (no external lights power) the LCDs are powered by circuit 296, power distribution. This is "keyed power" on when the ignition switch is on.
With the HL switch in Park or Headlights, the relay is energized and the LCD power is transferred to the HL switch dimmer.

Enough with the tutorial. Now we have to figure what's going on.

With the ignition off and the HL switch in Park, all the parking lights front and rear, and the dash lights are on , correct?
With the ignition off and the HL switch in Headlights, the HLs come on and all of the above lights go off, correct?

With the engine running and the HL switch off, the cluster LCDs are on, correct?
With the engine running and the HL switch in Park. all parking and dash lights, and cluster LCDs are on, correct?
With the engine running and the HL switch in Headlights, HLs are on and all of the above are off including the LCDs, correct?

Are all of these statements correct?

No, whenever the headlight switch is fully on or in the park position the taillights are off and the digital display goes dark and the rest of the dash lights including the radio and hvac system (only the speedometer is digital) the headlights work fine and the front parking lights come on although I just noticed I have a bulb out in the middle front. at one point the taillights did work but the display was still dark so it may be 2 separate issues, maybe its a ground issue in the rear someplace? sorry for the slow reply

Fuses have two test points on top of them. You can probe these test points with a test light. This allows testing fuses without unplugging them.
With the HLs on probe both test points on fuses 15 and 13.

x2 on fuse 15. I hooked some jumper cables backwards and fried a bunch of fuses and had the exact same problem.

I'll check it tomorrow and let you know I hope that's is

I checked every fuse in the box and all are working I also replaced the burnt bulb in the front. I cant help to think that this is a problem with the headlight switch, before I changed it the taillights worked but the dash still went dark now it seems both aren't working

Are you saying the test light showed power on both test points of fuses 13 and 15?

Did you print out the EVTM pages?

Yes both sides are testing good I also pulled them and they all look good, its very strange when the headlights are on the dash lights are off  accept for the radio eq and the clock, all the other lights work ie turn signals brake lights headlights hi and low, as well as the front driving lights, are the front driving lights and the rear tail lights on a seperate circuit.

I haven't printed them my computer is having issues downloading things

Yes, both the front and rear parking lights are powered by the same section of the switch.
The front lights are working and the cluster illumination relay is being energized.
We know the relay is being energized because the speedometer LCDs go out. The relay transfers the LCD power to the dimmer section of the switch and none of the dimmer controlled lights are working.

The front parking lights are powered directly from the switch and the rear lights are powered through fuse 15.
Since your test light shows power at fuse 15, the tail light problem is down stream from there.

The dash lights are powered from the dimmer through fuse 13.
Since your light shows power at fuse 13, the dash light problem is down stream from fuse 13.

I had you check both test points on the fuses because sometimes a fuse can be bad even though it eyeballs good.

So I will have a broken wire or bad ground

Check any brown wire you can get to at the tail lights with your test light.

Pull the ashtray illumination bulb and probe the socket contact. (I hope that is easy to get to)

Could be, but there is no common ground or wire for the two problems.

I do appreciate all the help I'll do more checking tomorrow

Diagrams

More diagrams

dude you are awesome thanks!

I know he is I swear to God I don't know what the world would've done without him

I haven't had time to look at it today but I'll be working on it tomorrow morning for sure

Ok so I tear out the instrument panel and start looking and testing wires with no luck so I decided to look at the fuses one more time just to make sure so I stick my head under the dash and start testing and they are all good except now I can see because my head is burried under there that number 13 & 15 fuses are missing completely so I pop them in and low and behold I have tail lights I'm going to put a heater core in it and put the dash back together and see if it all works

oops

Doh!!!

When all else fails always start at the fuse PANEL. No Fuse No power no lights SIMPLE!!!

Right I just don't understand why those were taken out, I think my buddy may have pulled them when we making the brake lights work, he replaced the switch for me he's a small guy and I'm on the larger size so it's hard for me to fit under there

Not a problem teal. Been there many times. But what people do not understand is they over trouble shoot a problem. Always go back to an issue if you just worked in that area. And when doing a trouble like a brake light issue always inspect the entire vehicle. And here is the BIG PROBLEM. Always bring a hand full of fuses with you when doing electrical troubles. Do Not take one from another spot and then forget to put it back in!!!!!!!!!!!!