Sadly, the presskit uses wording that has mislead many of us into thinking our beloved M3's would come with heated rear seats with the $5K PUP upgrade. Unfortunately, it appears that it was just poor wording on Tesla's part and that the M3 does NOT have heated rear seats.
T-SportLine confirmed the absence of heaters in this thread, and since they've re-upholstered their M3, I tend to believe them:
Vendor - World's First Customized Model 3: Project 3X Walkthrough
Telsa's misleading wording form website/ press kit:
"Premium heated seating and cabin materials throughout, including open pore wood décor and two rear USBs"
T-SportLine confirmed the absence of heaters in this thread, and since they've re-upholstered their M3, I tend to believe them:
Vendor - World's First Customized Model 3: Project 3X Walkthrough
Telsa's misleading wording form website/ press kit:
"Premium heated seating and cabin materials throughout, including open pore wood décor and two rear USBs"