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It sounds like you have a possible door switch problem, and the computer is thinking the door is open. you will have to check the rear switches and if the switches are operating ok, than it will be a wiring issue that you will have to locate or a bcm problem. be sure to make sure the grounds are good on each door. you will probably have to remove a few panels to locate the switches and wiring, and this will be a little time consuming.
If you have access to a diagnostic scanner, than this can make it a little easier by reading what the bcm is reading.
I have included a wiring diagram for you.
the grounds are located at the left and right kick panels. It will be a bit of time on your part to test each switch operation and grounds, but it is the only way to find the problem. if i had this vehicle come to me, this would be my next step. monitoring the door switch operation on a scanner, than removing the panels to access the wiring and switches.