I have just installed a Becker 4602 in my 1987 560sl. Looks great and sounds perfectly fine. BUT...no AM at all. Just hissing. I am guessing that one of the leads from the adapter harness provided by Becker needs to be attached to the one wire from the old radio that I didn't connect to because there was no obvious fit. Anyone tried this installation and had this problem? I'm reluctant to wire things together that aren't evidently intended to be attached together. Thanks.

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John C
1987 560SL 109,000 miles
2001 E320 103,000 miles
2007 E350 32,000 miles