04 rear cargo door always locked

I've got an 04 Tahoe LT, and the rear cargo door seems to automatically lock itself when I start it and put it in gear. Then when I shut it off and jump out to unload the back, the door is still locked.

It never unlocks itself.

I have the computer set to lock all manually and unlock all manually, so the rear door should never lock itself automatically.

So I either have to get used to unlocking all the doors everytime I want to access the rear cargo area, or program the computer to "unlock all doors in park". Either way it's a workaround for a problem I shouldn't have.

I know of another 04 Tahoe LT with the same issue - anyone else?

suggestions?

Thanks