rear defrost

Hi all, My 01' Tahoe's rear window won't defog and its driving me crazy. When I measure voltage at the rear I'm only getting 5 to 6 Volts, shouldn't I be getting around 12v? The fuse/relay are good but I've also noticed my pass side mirror doesn't warm up either. Are these connected...

Both fail at once. Replaced wiper motor, helped for 3 days. Took to shop, they replaced the wiper motor, worked for 5 days. But both defrost AND wiper stop working at the same time. Fuses OK. There is not common switch. Is there a common ground anywhere? No part of the defroster grid is damaged...