Hi, Id like to say hello to everybody, I am new to this site!

I have a 89 300e w124. Last winter, I had an issue with my driver front door lock. I can no longer lock the door with the key, I can only lock by pushing the plunger. Even then the alarm engages, but the door still opens from the outside. I finally took it to the dealer, they said that my lock cylinder was bad. Ordered, (400.00!) Just to find out that is not the problem. Does anybody know what could be the problem? I recently found a door latch on ebay, but I cant see how the lock would disengage the outside handle. Rebuilt my whole engine from the block up, and im stuck on a stupid door latch.

Thanks