I am not sure I am 100% clear on the yellow wire. You said not to attach it to anything which is clear enough but then you said " In the 1999 Tahoe that wire in the seat connector was not used outside of heater electronics yet". That statement would lead me to believe that the yellow wire was used for heater electronics but nothing else yet. Are you saying that the factory vehicle side connector won't have a yellow wire in it, that the seat side connector has the yellow and when the two connectors are attached together, on a vehicle that came equipped from the factory with heated seats, the yellow male connector on the seat side will be attached to an empty female slot on the vehicle side?
By the way, I really appreciate all your help here. I just rigged up the driver's side seat with a battery and some wires in my garage and it works fine. I am going to try the pass side now.