What is the position of your dash antenna switch while you are testing? My shop manual diagram, for my W126, which may differ from your 1990 model, shows no direct connection to the antenna from the stock radio. Rather, the power to the antenna ciruit board is from the dash switch, antenna plug pins 5,6,and 2, and from the fuse, pin 1. Pin 3 is ground, pin 4 is unused. The 'trigger wire' from the radio goes to pin 1 on the dash switch.
If someone has wired in another radio/head unit, you may need to check the wiring to and from the dash switch, as it must be wired correctly to 'talk' to the circuit board in the antenna. I never found the switch to be anything but a nuisance, as I always want my mast up as high as it can go for best reception. The switch and antenna circuit board are designed to allow momentary 'bumping' to a selected height less than maximum.
Sorry, but I really don't know what exact steps to take to confirm the circuit board is working correctly. With the switch in the 'max' position, the mast should go full up when the radio is turned on. In the center position, it should go up about 30cm, then you can 'bump' the rocker to raise or lower it in increments. In the 'off' position, the mast will stay down, or retract if it is up.

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1986 560SL
2002 Toyota Camry
1993 Lexus