I had a factory fitted tow hitch hooked up to my XC60 by a Volvo dealer. The wiring loom etc is all std Volvo and the plug is wired as a standard (AU) 7-pin setup. When any resistance at all is placed on the plugs the system malfunctions and no power is passed to the trailer... Symptom: Engine on, Indicator on [Indicator flashes at normal speed]. Rest circuit test lamp on Indicator pin (eg #6) [Indicator immediately flashes at twice normal speed]. Indicator needs to be turned off and turned back on (with no connections to tow-hitch plug) in order to resume normal operation.

Can anyone offer advice as to cause???