I had the same problem with my hitch/plug setup. Whenever I put on the brakes the trailer would go to full lockup. I had my hitch installed at the dealer and it appears that the factory setting wires the brake lights share the same wire that the trailer electric brakes use…or something. I printed out the install instructions from Mr. Wolfgang’s site and took them to a hitch specialty shop. They installed a brake controller no problem and it worked fantastically after that. I had some other strange towing issues…..

When towing up hill (hard) for a period of time the low oil indicator would go on. Then after a while it would turn off. I even had it show low oil show -2 oil warning in the odometer section then 5 minutes later it showed oil hi!!! When I checked it that night it was 2 ticks off of full. The temp gage would also start acting funny jumping from 80 to max back and forth very loosely. When it did this my AC would shut down. Even with these strange occurrences it towed solid with good power 4200LB trailer with a 98 ML320 245,000KM on the OD.

Have fun hope this helps.

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1998 ML 320
BIG Sound