driver's side door lock...
 

O.K. just got my 300E back from the shop (for other work)
The drivers side door lock does NOT UNLOCK-it isn't vaccum, or a line or any of that-it is the lock cylinder itself (and i PAID for that diagnosis-like I didn't know it-) Anyhow-the new cylinder-keyed to the existing key is $190-plus about $150 for installation....Now-I am thinking i need to fix this-I work late nights, and i would hate to be unable to unlock my car quickly in an emergency (I currently unlock it at the trunk)-question-can I use the PASSENGER side door cylinder-or will this not work (Hey I don't need the passenger side to unlock with the key..)
AND how difficult is this job??(how do you remove the door panels?) I am fairly handy-but i do more home-style wood-plumbing -electrical work-and NOT a whole lot of mechanical work -except for oil changes
Thanks
laura

Older model(my 83 300SD) you can actually work on the key cylinder. I pulled off most of those problem shims?(little metal rectangle on the insert) and it works fine. Newer model(89 300E) is impossible...I installed a remote control instead.

In terms of price: Yes, sadly this is something best done at the dealer, and yes, the entire assembly must be ordered, hence the $190 part. The labor estimate seems extreme -- I would expect 1/2 job for an experienced tech.

In terms of swapping the innards: Most of the work is in getting the door skin off. It is a reasonable DIY job -- a neat trick to swap the innards -- I don't know if it would work. On my 190E, the passenger side does not unlock all doors -- not sure if it is broken, or by design, nor do I know if it is the same deal with W124. Hope I answered your second question, anyway, Andrew Seidel.

i read that you will be able to use your pass side cylinder. i also read that you will have to mount it upside down which moves a water drain hole to the top, potentially causing problems down the road. shop around, the first quote i got was $159. also, if you do a search, there are comprehensive descriptions of how to take off the panels - it actually looked a lot easier than i expected. good luck and keep us posted - i already had a run in with my alarm system caused by the same cylinder/switch assembly...