Ok so having got hold of my cable, I now need to run a wire to the socket at the back of the car to supply power to the caravan fridge and/or charger.  To be done properly, I think I should take a feed from the battery, wire it into the main fuse box under the bonnet with a 30A fuse, then run it to the back of the car.  The only problem with doing this is that I cannot poke a wire into the car through the bulkhead without dismantling half the car and dash (I think) so I’d have to run the wire from the fuse box under the car to the back.  It’s protected by the underpanels, so wouldn’t be totally exposed, but there are no other wires that I can see outside the car like this so it makes me a bit nervous.  Fuse would be right next to the battery so if the wire under the car gets wet and shorts out I’ll be protected.

The only other option is to piggyback the fuse box inside the car – there is one thick wire which I think is a supply from the battery but it has to supply everything else from the fusebox and I don’t want to overload it.

Anyone?  TrailRat?

I’d try and run it inside. I’ve never had an issue finding a suitable hole in the bulkhead. Usually find something in the footwell or behind the glovebox. Poke a rigid cable tie through then tape the cable to it once you have found the other end. Might need a couple of tries. I wouldn’t be keen running it externally but its not the end of the world if it has adequate protection I suppose.

Use the same path as the wire from the battery to the internal fuse box. Dashboards tend to look nice from the drivers seat, but if you contort yourself into footwell, you can see cables, cooling pipes, pedal controls and all sorts coming thought the bulkhead back there. When I’ve done similar in the past I’ve run the wire under the trim that covers the carpet to door jam joint and on into the boot.

Use the same path as the wire from the battery to the internal fuse box

Hmm.  Internal fuse box is on the opposite side..  I’ve had the dash off this car and the bulkhead is a looong way back.  I might take a look under the engine fuse box see where that goes into the car.   I can’t just use any old hole though, I’d have to be trailing wires all over the engine past a lot of hot stuff.  There needs to be a hole behind the interior fan which is behind the glovebox.  Might be able to get in there.

There’s nowhere in the boot to connect to?  I’m certainly no autoelectrician but I watched one do mine when the towbar was installed and there was certainly no attempt to feed wires through the car to get to the battery.

Tell me you have a VW mol old boy…. Our Touran had a total lack of wiring ‘in case’ of tow bar, rear speakers etc. It was VW penny pinching at it’s finest.

There’s nowhere in the boot to connect to?  I’m certainly no autoelectrician but I watched one do mine when the towbar was installed and there was certainly no attempt to feed wires through the car to get to the battery.

Ah hah!  Yes, this was originally done by an auto electrician.  He wired in the trailer module with the harness it came with, then he clipped a nice thick wire onto the nice thin one that’s meant to supply the module itself and from that to a 30A fuse then to the voltage sensing relay and to the socket.

The problem with this approach is that when you start the car, the voltage goes up to 14V ish, the relay clicks on, 10A starts flowing and because of the long cable run from the battery to the relay, the voltage at the relay drops back under the threshold and it clicks off again, at which point the voltage at the relay goes back up to 14V and it clicks on, and so on.

I always wondered why the caravan fridge didn’t really work, I thought it was a problem with the van’s wiring.  But no – when I adjusted the relay to be always on the fridge worked really well and got really cold.  Reading up, this is a common problem if you install the relay in the boot – which is why it needs to be placed up front close to the battery.

Well. Turns out the trailer module had died. Couldn’t get a replacement so bought a new Westfalia kit.

Anyway, still had to run a wire to the engine bay, and it turned out to be the easiest part of the whole job. For reference, this is a B6 Passat. Behind the battery to the right of the ABS pump the main wiring harness goes into the car via a big fat grommet, but VW have left a load of small sub grommets that are blanked off, apparently for this purpose. So I snipped the end off one and poked in a wire and they’ve left enough room so the wire went straight into the car really easily. Taped the trailer harness wire to it and pulled it back through easily. Thanks VW!