I have same situation. Hirschmann generic replacement on shelf, original mast doesn't extend.

I just leave the original in up position and avoid car washes that would scrunch mast.

From what I recall, the generic Hirschmann has slightly different wiring. And, I have replacement radio so activation of antenna would need to be figured out.

Will only change if fixed mast gets bent.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5