I have a 96 SL, so I don't have a power trunk opener. I didn't know the 129's even came with those. Sorry I can't offer much, but I would be very careful if you mess around back there, some have messed up the lock and not been able to open their trunk at all, that is a mess on that car since there is really no other way in the trunk. I would check the owners manual first to ensure it is operating properly and then if not take it to the dealer. I am pretty good with my 129, but when it comes to that rear lock, I don't mess with it, since it can goof up and leave you locked out for good.
Brian

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Brian W. Heitman

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