Often times a towball shouldn't affect them. But it can. Some can be programmed to ignore it, some can't.
A VERY common fault I see is water getting into the sensors. Not a great deal you can do either. You may be able to pop them out and hit them with a hairdryer. But it can happen again.
If in the event that this not the problem, then it's usually only one that has failed. The control module will be located in the boot somewhere and just needs you to unplug each individually until you find the culprit. The system will work just fine without it. Some will give you a quick error chirp to alert you if one is unplugged and the continue working.