Well John, I guess i'll never get to the bottom of this one.
When connected direct to the battery, and the switch earthed to the -ve terminal, and switched off, there were no volts showing on the earth side.
When on, we got the 12v we expected with the switch illuminated.
As soon as it was connected to the 4m cable inside the vehicle, I got the short circuit through the Earth lead.
Tested the cable and no faults in that. So the mystery was never resolved.
We solved the problem by, erm, taking out the switch altogether. I have put a 10A inline fuse into the live cable and then run it all to a 12v boat socket at the rear, which is hidden near the floor behind the kitchen unit.
Into the socket I have plugged a 4 x 12v socket adaptor from Halfords - the one with a remote switch block, with an illuminated on/off button for each of the sockets:
http://www.halfords.com/webapp/wcs/stor ... yId=70567#
So I can now isolate everything which is plugged into the main 12v socket.
In the 4 sockets I have:
1. Water pump from kitchen unit
2. 120w Inverter feeding a grey 3 pin 240v socket mounted flush into the side of the kitchen unit.
3. A neon light to go in the cupboard of the kitchen unit
4. A 12v plug feeding a grey 12v socket mounted flush in the side of the kitchen unit below the 3 pin socket.
The remote button switch is mounted below the sockets on the side of the kitchen unit, so you can see at a glance what is switched on.
Looks like it was done by a professional
The two good things about this system is that I can independently switch on/off each of the 4 sockets. This means the inverter is only running when I need it. As its fan makes a humming noise this is quite useful, as to have it on all the time, even when not being used, would be a nuisance.
The other good thing is that I can now take out the kitchen unit from the vehicle, and just have to unplug one plug. All the electrics are attached to the back of the kitchen unit so can be removed with it.
Once out, the rear seat can be bolted back in, leaving just the 12v socket on the wall by the floor under the seat.
I'm very happy with the way it's all turned out, and even happier that I only set fire to one wire last night, and not the whole Bongo
Out of interest, I got the grey 3 pin and 12v sockets from a company called C.A.K Tanks (
http://www.caktanks.co.uk), who do a whole range of matching sockets, switches, guages, TV/satellite sockets etc, and then you buy a little frame and cornice to fit round them and finish them off neatly. You can get singles, doubles or triples.
When fitted to the side of the grey kitchen unit they look like they came with the unit they are such a good match. And all the gubbins to run them can be screwed to the back of the kitchen unit hidden out of the way.
I would post some pics of how the sockets look if I could work out how to do it
Thanks to everyone who has given me advice on this one.