I have the same problem except with my drivers side door...it wont lock or unlock at all. I dont know if its the lock itself that is broken, but my car has been without power locks for a few months now. Ive been reluctant to replace the vaccuum pump in my truck because its very expensive for my model (pre 85). For some reason the post 85 ones are much cheaper, but I dont know if they are compatible.

__________________
1983 300SD White with Grey interior

my drivers door lock did the same.. i sprayed some white lithium grease into it.. stuck the key in turned it to the left and right and it broke free.. huzah perfectly working door locks and trunk fuel door. knock on zebrano wood

I have been able to reach the unlocking mechanism that is connected to the vacume solenoid in the trunk. I pushed it as far toward the lock( left) as i could. The button wont go in at all. I think the latch is jammed. The button pushes in easily when in the locked position, but when in the unlocked position the button wont go in. I have tried a rubber mallet, lifting and pushing down on the lid many times over. no avail. I called a locksmith. he couldnt help. Didnt even want to try. I will have to call around and see if anone else can help. Im stumped. I am kind of affraid to take it in for i think the repair may be expensive. I could try gong through the tail lense, but if the latch is jammed, i dont know if that will help. That may just add to my bill. Maby I can get to the bolts that hold the trunk lid to the car and get it off that way. Doubtfull. Looks like it is going to be a little while before i figure this one out.

What is happening with the trunk lock???

Note:
Here is a link to the other thread.

Help! trunk wont open Tools inside No cable
http://www.peachparts.com/shopforum/tech-help/152712-help-trunk-wont-open-tools-inside-no-cable-post1163205.html

the mechanical lock? mine didnt work so i sprayed some white lithium grease in the keyhole stuck the key in jiggled it around and viola it opened and locked etc

C.DONER,
I don't know if this will apply to your car, but I had the same problem on my 84 500SSEL. My car has a factory alarm system that will lock the doors and also lock a secondary trunk lock. This secondary lock is electrically operated by the alarm controler located under the passenger foot rest (or what ever you call it). I just went out and tried opening the trunk with the key while it is locked by this secondary lock. Well, there is no way to open it until the alarm system is disabled. The key turns CCW but will not push in in that position. Disable the alarm and then the key works as it should and locks or unlocks the trunk. In my case, the alarm switch in the drivers door went bad but the passenger side would disable the alarm. Try unlocking the doors from the passenger side. You may have to lock the doors from the passenger side and then unlock from the same door. See if that works. If not, let me know and maybe I can look up the wire colors for this trunk lock and tell you how to activate or deactivate it with jumper wires.

__________________
84 500 SEL (307,xxx miles)