the auto door locks work sporadically. Sometimes they work and other times no, on 2008 Buick LaCrosse

the trunk always open so we know it isn't the remote.

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2 answers
Have you checked the motors for the door locks? They are a weak link because they are used frequently. You may want to go to your local auto parts store to pick up a motor manual for your car. It will show you step by step instructions for removing your door panels and anything else you may need to do. Hope this helps. Goodluck
door lock actuators are poss going bad they are known for that