Join Date: May 2012
Location: East Saint Louis Co
I would start yanking fuses, (One at a time), until the symptoms quit. e.g. with the trunk open and the third brake lamp glowing, pull the i.o.d. fuse and see if it goes out. I haven't taken a look at the schematics on this but, I'd be tempted to say you are picking up stray current from somewhere.
I suppose it is all possible that there is or was an aftermarket alarm / remote start system installed on this car and has become naughty. The center stop circuit voltage may have been used as a signal for stop light detection.