I've searched for an answer to my problem for a solid hour now and nothing seems to fit my exact situation. Here it is:

A friend of mine has a recently purchased 1985 300D with a Hirschmann antenna (plastic cover) that has a broken antenna mast. I ordered a replacement mast on eBay (toothed feed cord) and installed it today. It extends fully but won't retract. The motor runs and the shaft begins moving down but then it hangs up quickly. I can push the mast all the way down and when you turn the radio on again (aftermarket) the antenna fully extends just perfectly. I'm puzzled. Why would the antenna extend and not retract?

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Diesel Fitter
Lily Von Shtupp: 1983 300D w/199k
Communication Breakdown: Chris T.'s new WFMU Podcast

It sounds to me like the relay that triggers the antenna is somehow defective...When you turn the radio on the DPDT relay sends the antenna up and when you turn the radio off it removes power and the relay sends the antenna down...Somehow the power is missing on one of these positions of the relay. I would also check inside the antenna for any mechanical problems that might be obvious.

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/car-audio-multimedia/125099-123-install-extreme-dynamat.html

Did you remove the broken part of the shaft? I had a similar problem and there was part of the plastic shaft still in the unit. Had to cover the workings to get it out.

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1977 300D Lost coolant while someone else was driving
1983 300D Can't run without oil
1985 300SD (gone but not forgotten)
1990 300TE 4matic Sold
1991 Yamaha Venture
1975 Kawsaki 250 triple
1974 Honda 200CL
1951 8N Ford
2008 Wildfire 650C