Bought a 1998 Mercedes diesel after half a lifetime of 80's diesels, just in time to appreciate it starting at -20. Anyway I'd been using the flat spare key because the regular key wasn't working.
It's the oldest style lock.

So I picked up a similar key off ebay and switched my key chip onto the functional board.

Sorry I didn't take more pictures of the soldering process. I sort of kept rolling once I was going. my first time IC soldering and I used the method described here
Salvage surface mount components
I used a short piece of 14ga solid copper wire(16ga would probably be better) and a scalpel to pry underneath and it actually worked really smoothly. Desoldering one side at a time.
I've tried the key in the ignition twice now and it's success rate is 100%, up from 0. The radio locks don't work. I think there might be a frequency discrepancy.