Towbar Electrics and Rear Fog Lights

Following a change of towcar which came fitted with twin electics I have converted my van connections back from a 13 pin put on by the previous owner to its original design of 12n and 12s plugs. This is on a series 6 Pageant 2007 model. All works fine with one exception and at least I have regained the battery charge function that was missing with the 13 pin set up. The rear fogs do not work with a straight switch on and I have checked that my plug connections are fine. The car is a Honda CR-V 2010 model but the dealer was unable to say who fitted the towing equipment although it appears to be the Honda approved kit as it is a Bosal bracket. I suspect therefore a dedicated harness has been used. Any thoughts here please before I have to go to the expense of having the car electrics checked out. I am happy to fiddle with the van side but car sparks are another subject these days. Long gone the days of twisting the wires together and checking the earth was sound! Geoff