My antenna only goes half way down. If i press the button to Off it still only goes half way down.

Check the mast - it might be binding, requiring lubrication or replacement. They aren't very expensive.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

The mast is a cheap easy DIY.

PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1989 Mercedes Benz 420SEL
Part: Antenna Mast
Note: Toothed
1105 mm

Z4010-16715 Antenna Mast Hirschmann IN STOCK $37.73

Z4010-16715 Antenna Mast APA IN STOCK $34.25

http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1YE1FF2PQ1YU1DLE1V&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1989&product=Z4010-16715&application=000026968


PART NUMBER SEARCH RESULTS WITH PRICES
Vehicle 1989 Mercedes Benz 420SEL
Part: Antenna Mast
Note: See also # Q 482-0004.
Smooth
915 mm

Z4010-22930 Antenna Mast APA IN STOCK $34.05

http://catalog.eautopartscatalog.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1YE1FF2PQ1YU1DLE1V&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1989&product=Z4010-22930&application=000035640

does it continue to make a grinding noise once it stops moving? if so then as everyone said its an easy fix to replace while you're at it also replace the inner and outer seals for the antenna hole in the fender. if its not trying to go down and more once it stops remove the mast and then re- feed it in to see if that helps. when i put my new mast in i thought i had the wrong part number because it wouldn't go down all the way, turns out it had just slipped off the retractor and needed to be re- fed

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E30 B25
126.036
123.130 with 616-Frankenbenz (sold)
124.030-Greyhound (sold)
124.030-Schnauzer (sold)

DERP DERP

I just took the mast apart, and found the second stage was bent. I fixed the bend and put it back together with some oil on all of the parts,and so far it is working.
Thanks for your help.