It takes a bit of wiggling to get the assembly out. Reach up and push the instrument cluster out of the dash to make more room (it should move about 2-inches without having to disconnect anything). With the clamp loose, move the assembly away from the steering column and up into the space where the instrument cluster was. There is barely enough room to get it free, but you shouldn't have to force it, much.

Edit: I believe there is also a button someplace on the assembly near the clamp that will free it up. I don't remember exactly, it's been a while.

I think that is the button you can see in the photo:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ42E81BDCQG&year=1983&make=MB&model=300-TDT-001&category=All&part=Ignition+Lock+Housing