Sorry for the delay guys, I wanted to make sure I heeded everyones advice before replying.
I hope everyone is having a relaxing holiday.
This might be long winded....my apologies

-The PO gave me three keys with the car.
1. The valet key. (this is the only key that worked in the ignition)
2. The master key. (looks worn down doesn't work in ignition)
3. A key to a different mercedes...... LOL! (AFAICT this key doesn't work anywhere on- ----the car. It also looks different than the other keys.

A while ago I had a locksmith make a new copy of the worn master key for me. Cost me $45, but it should outlive me I suppose....

This key works perfectly in both doors, ignition, AND the glove box. AFAICT any key that works in the glove box should also work in the trunk??

But it doesn't, the key slides in perfectly but it won't turn. I've sprayed a medley of wd-40, and pb blaster into the key hole. The locksmith even tried with something he had.....

I will say, during these soakings some black sludge did come out.

Last night I sprayed wd-40 into the keyhole and let it sit overnight. I sprayed more in this morning and before I wrote this sat on the trunk with a morning coffee and worked the key around for 10min. No go

There is a vacuum/pressure controlled actuator (rubber diagram, plunger.... you get the picture) that pulls or pushes a threaded steal rod that goes into the locking mechanism. The PO appears to have unthreaded this rod from the actuator, so it's just poking out of the lock mechanism unhooked.

I would like to fix this soon, BUT... shouldn't the mechanical lock turn just fine disconnected from the actuator???
I also tried turning the key while I pulled/pushed the threaded rod to it's two "set" points. I was hopeful on this one but still nothing.

I'm thinking my next step is to remove the actuator (have to anyways) and maybe the lock itself and go from there. I'm thinking there is some cobblery involved, but we'll see.

Thanks for the input, keep it coming.

minsk- are you expecting snow? I just checked the weather and looks like maybe 5-6 inches for here before the end of the week.