Check where you're plugged into the cigarette lighter, that's probably where the bad ground is. If there was no problem with radio before, I doubt that the condenser or lack of it is a problem.

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'83 240D (for sale)
'84 Harley FXRT
'85 300CD Turbo (Sold)
'85 500SEC (for sale)
'04 Dodge Dakota Quad
'06 Harley FLHTCI
'97 SL320

I previously had a Sony unit that would do the same thing on the AM band, so I'm betting the condenser is the problem.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.