Sorry to be the bearer of bad news . . . I've had the ashtray mechanism apart before. If the ashtray won't stay closed it's because a little plastic pendulum type part inside has broken.

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Unfortunately, the mechanism is not repairable unless you can come up with a replacement pendulum piece. I've had to buy complete new ashtrays (actually bought the whole lower section of the center stack as pictured which included a working ashtray mechanism) and just replaced that whole section.