I did this. If you study the circuit diagram in the manual and observe the terminals in the fuse box, you should be able to work out which terminal takes the 12V supply from the towcar cable. I just connected a on-off-on 3 way switch onto here with the common termnal going to the fuse board, one switch terminal to the 'car' input and the other switch terminal to the battery, via a suitable fuse, of course. With this set up, I can switch between towing vehicle input, leisure battery input or neither. Remember that having 12V switched on disables the ac input so if you are on mains hook up, you must switch the 12V supply off! The 12V supply powers all the lights and pumps but not the fridge. (This on a 320GT around the same age as yours).