Tried a number of Vauxhall dealers for supply of a TRAILER OUTPUT PLUG so that the 12 supply could be connected to the tow bar universal relay, but nobody in Vauxhall had ever seen this plug and had NO IDEA where to pick up 12volts.
I then decided carry out the usual method of cutting a chunk of insulation from the 12 volt wire to the rear fuse box and soldering on a wire ( see photo ). The 12 volt supply wire is very thick and to do a good solder joint the soldering iron must be very hot, I used a 48 watt iron which was just right for soldering to this thick wire.
Also spoke to the designer of the universal relay, he told me it is perfectly safe to leave the relay permanently connected to 12volts. I was going to fit a switch to switch off the 12 volts when not towing but now didn’t bother to do that. Tested all working OK.