He's probably talking about the ACC amplifier. When the AUX water pump fries, it takes out the amp with it, or vise-versa. You can take off the glove box inside by taking off the small clips inside the glove box. It's a small circuit board in a black plastic housing. If it's fried, the ACC won't work. You can check if it's toast by looking for burned contacts on it, IIRC rebuilt ones are about $100, you can try to fix it by re-soldering all the joints.

To remove the ACC amp, remove the glove box inside by prying the clips off, take off the amp by loosening 2 screws and unplugging a harness. Also you can test the amp with a multimeter, seach for the exact procedure. New from MB the board probably cost $500 or something rediculous, but rebuilts are good.

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1980 240d
1999 SL500