Underneath the caravan there is a connector into the ATC , thats the best way.
When you its giving 'problems' what do you mean? as incorrect brake shoes/cable/handbrake adjustment is the main cause , the ATC when it goes through it self check, if the push moves too far it signals a fault , which is not the ATC it's the brake adjustment. also if you tow like an idiot if it detects too much sideways motion it will also signal a fault and lockup