The tuner from your W126 probably won't work in his W124: It probably doesn't have the internal amp hooked up. At least that's how mine is.

If you have any questions about the system let me know... I spent a lot of time un-hacking the JVC a prior owner had installed in my car.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.