[Zach@ExtremePerformance]

The car is being bathed by the signal from all overhead GPS satellites at any given moment. The RF signal from them is unaware of whether or not your car has a GPS receiver in it, nor does it care; it hits your car, your neighbor's car, and everything else both vehicular and non-vehicular underneath it equally. So that's out.
The entire headunit does create a small amount of RF; it has a switching power supply so that's an inevitable result. It is not, under normal (or even the vast majority of abnormal!) situations, anything even remotely enough to cause issues with the TPMS. If a winding in the power supply has gotten loose enough or whatever to where it's messing with a sensor in the BACK tires...other computer modules (many of which are much closer to the radio) in the car are going to be acting up as well. So I think we can safely eliminate the radio with a gentle application of logic and common sense.
It IS possible that the sub amp is generating enough RFI to cause issues with the TPMS (speaking of which, did you get the memo on using cover sheets for your TPMS reports? I'm gonna go ahead and get you another copy of the memo...), but that does raise the question of why it's only just now causing issues. Not totally impossible, but an issue somewhere else is more likely. The dealership may say "it's not the sensor, it was working before" but you can point to everything aftermarket and say the same thing.