You are correct, it is in the door lock. There is a small metal switch, black, at the base of the lock which you can push toward the outside to 'close' the door and turn off the lights. To put the switch back in the unlock position, just hit the unlock switch or pull on the inside door handle. The door will not close unless the switch is in the unlocked position.

So... you have an adjustment problem with the lock because if you slam the door, the micro switch turns off. The latch and the striker are both adjustable, if you have a good torx driver.

Klaus
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Volvos should be saved from the junk yards, not donated.