I had a similar problem in my 300D turbo about 2 yrs back, I disconnected the battery, sprayed some WD40, inserted the key in and used my homedics body massager and hold it against the key and let the vibations run in thru the key and use the free hand to wiggle the key after few attemps it did turn, I also had another key, that worked little better soI started using that key. Also sprayed a lot of graphite lock spray and It has been working since then. I still spray the graphite spray once every 6-8 months just to be sure. and touch wood no probs.
I dont know if this will do the job for you but its worth the try.
uSE A NEWER KEY IF YOU HAVE IT IN SPARE.

Good luck

MVK

__________________
One penny saved is three penny earned.

1985 300D Turbo 179,000miles