Can the radio be turned on when the key is out of the ignition (off)? If so then the radio is not powered by Fuse #4 (F4) and you need to look at the other sources for the fault (cigar lighter seems to be the most likely culprit at this point). F4 is hot when in Accessory, Run or Start, but not when ignition is off. F4 also controls the glove box light, the passenger mirror and front heated seats if installed. The antenna motor control relay is powered by F4 through the original radio, but the antenna motor itself is powered by F2.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)