I'm going to be installing an Auto Meter shift light/digital tachometer:http://www.autometer.com/black-dpss-tube-level-1.html on my Thunderbird. The shift light has a three wire hookup: red to 12v switched ignition, green to coil negative, black to ground. I'm running the green wire to the coil negative wire and the black wire to a chassis ground. Could I just run the red wire to the positive side of the ignition coil? After all it is a 12v switched ignition source. I figure since I'm running the wire to the coil and already there that I might as well tap into the 12v positive side of the coil. That should work fine correct? I helped a buddy install a MSD shift light on his 89 Mustang several years ago and we followed the wiring http://static.summitracing.com/global/images/instructions/msd-8952_frm28827.pdf that had us connect the green wire to the coil negative side, the red wire to the coil positive side, and black to chassis ground. It worked fine.
I think I'm just going to get the power from the coil positive side wire and see if it works. I don't see why it won't.
I don't see any problem with wiring it the way you want. It will work fine and I expect these two wires are glued together directly from factory.
I'll give it a whirl after all the snow melts. I think it should work fine.