I wanted to know if theres any aftermarket radio that can be installed in the SLK, I've checked a few obvious places online, and they do not have the vehicle researched.

Please check this website: http://www.crutchfield.com/S-O5qAUgoWScy/

Yeah that was one of obviously places that I already checked and called. And they didn't have it then in 2006.
It came down to who had a harness that would be compatible. But thanks anyway..