Check the POLARITY of the voltage at the coil with a good DMM with the key ON and the key OFF but with the lights on. I suspect that the polarity will be reversed and the voltage relatively low with the key off but the lights on.
Ignition off, lights off 0.0V Ignition on, lights off +12.1V Ignition on, lights on, +11.9V Ignition off, lights on, +10.9V
Now we are getting some where. So your pink/yellow are tied together, so what I would do is dissconnect the yellow from the starter solinoid. This should have absolutly nothing to do with it but then again you shouldn't have any voltage at that point with the key off and lights on.
Do you have the colored wiring diagram PDF that fits on one 11x17 sheet of paper? If not send me your email address and I will send it to you.
With your ignition off and lights on and your volt meter set up to watch for changes I would start to disconect things under the dash untill your voltage drops to 0. disconnect the ignition, disconnect the gauge pluge, disconnect the light switch, disconect your reverse light switch at the shifter. Something is back feeding your pink wire under the dash, and that pink wire feeds a lot items under the dash.
