I have the same problem with my 2005 Envoy XL. The rear hatch tends to not open in cold, wet weather. For example, I live in Cleveland and the rear hatch has not worked (meaning the hatch doesn’t open, nor does the window on the hatch, nor does the rear wiper, nor does the rear wiper washer.) However, in the summer, the hatch will open. It’s not guaranteed, but the likelihood of the hatch (and other parts) working in warmer dryer weather is greater than the likelihood of it working in wet, cold weather.
The key fobs, on the other hand, don’t work all the time either. (Both fobs) work sometimes and sometimes they don’t. When they work, the fobs will unlock, lock, and panic the vehicle. However, when they don’t work, it is like a fob was never provided. The key has to be manually inserted in the driver side door lock and then the doors must be unlocked from the driver door.
Very strange issue. The weather factor makes me think that it is a wiring issue. HOWEVER, the fob issue and the rear door issue are separate, meaning sometimes the rear hatch opens and the fob doesn’t work, sometimes the fob works and the rear hatch doesn’t, sometimes both work, sometimes neither. GM dealer (on multiple occasions) has told me they have no idea what is wrong. Because it is such a tempermental issue, it’s hard to show the dealer. Any help would be appreciated.