I understand there is a separate fuse behind the radio and you need special tools to take out the radio. Most of the tools I've seen are for 1992 and up and fit into slots in the radio. What kind of tools do you need for a 1985. It's a Becker radio.

www.metraonline.com

They generally have helpful hints.

You could try and get in touch with the company.
http://www.mbusa.com/mercedes/contact_us/overview