I didn't know what the F125 switch was, so I had a look at audipages A8 F125 Transmission Switch replacement.
Looks like a lot of work with it being so inaccessible.
Good luck with the repair
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Johannes
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Audi S8 -99, Pearlescent, Alcantara/silk napa leather, Burr walnut insert, Alcantara roof lining upper pack
A8 32V engine
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Jeep Grand Cherokee -98