It could be the actual harness box. Based on the time frame, you probably have the old style harness box with a mechanical relay. Honestly the best path forwards is to order a new Q4 harness kit and the coupler. This will get you the new style harness box which is solid state instead of...
There isn't one. 4.0 was the only one we did that for because there were so many issues with the default model. But I've had good luck running Cool People