This is a quick guide for those thinking of using the plugs and wiring from a donor Scenic to connect towbar electrics to the existing 'extra' socket which is already on the rear light clusters of the Scenic / RX4.
The benefits of doing this type of installation is that it is cheap, simple, discreet and can be disconnected easily if required because it uses plugs, not directly soldered joints or splices onto the vehicles wiring loom.
NOTE: This was performed on a 2002 Scenic RX4
The donor vehicle was a 2002 standard Scenic. The towbar was previously fitted and the electric socket was also put in place with the cable poking out of the plate it's attached to (you don't wanna wire it all up with out threading it through this plate first!) Total cost about £3 (cost of parts from scrap yard).
ALSO NOTE: I am not a vehicle electrical engineer but have fitted several towbars and electrics to various vehicles which have given years of reliable service.
You will need:
Two rear light cluster connecting plugs and wiring from a donor vehicle with about 750mm of wiring still attached.
A soldering iron and solder.
A multimeter or descreet bulb circuit.
Some electrical insulation tape or shrink wrap.
A torque head tool to remove the screws of the plastic panelling in both your vehicle and that of the donor.
A small flat head screw driver.
Some strong string or cord.
Some means of cutting the wires and stripping back the insullation.
A light (small torch is good) to see into the rear light access aperture of your vehicle and other nooks.
A length (500mm is ok) of stiff wire or small diameter metal bar (something like a very stiff metal coat hanger).
A diagram showing wiring connections to a towbar socket . Here's a good one with buzzer -
http://www.just-tow.co.uk/towbar-fitting.html
On an RX4, the bottom lip (the edge nearest you when standing at the rear of the car) of the luggage compartment is covered by a plastic molding, held in place by 5x torque head screws. The screws themselves are covered by circular inserts which can be prized out with a small flat headed screw driver.
You may need to losen the rubber sealing strips grip to remove this, but once released, you should see the painted metal it covers. Unsure of what covers a standard Scenic rear lip but you will need access to beneath.
This newly revealed surface will have a series of round and oval holes already present. Using a small torch in a nearby hole, peer into the far left oval and look for a round depression in the surface below.
Push firmly through this underseal 'skin' (it's stretchy) with your stiff wire untill it pokes out underneath the car. You will probably have to feel around for it.
Attach a length of strong string or cord to the stiff wire with tape, then draw the string all the way back through to the oval hole with the stiff wire leaving some string dangling below the car and detach the string from the stiff wire.
Now, with your stiff wire bent appropriately and pushed through the central hole of the painted surface, hook the string in the compartment and draw it out through that round central hole.
You can now attach the towbar socket cable to the string and draw it all the way back to the round central hole. Leave it sticking up to trim back later when the other wires arrive for connection.
The donor wires attached to the donor plugs will be bound up in tape possibly with other unwanted connections, so you will need to carefully separate them and may need to add length to some strands (use the wires you remove). After creating two lengths of wires with their plugs ready to go (i.e. 6 wires per plug) you can install them into the vehicle.
How you route them will depend on your vehicle (standard Scenic or RX4) and your sensibilities toward drilling your car or your trim. I chose to drill 8mm holes through the underside webs of the plastic molding. I figured I could replace this molding more easily than fix drilled bodywork if things didn't work out and I don't like drilling bodywork as it can become a starting point for corrosion. Drilled holes are also less likely to create weak split points in the webs than cut channels.
Thread the re-bound donor wires down from inside the light cluster cavities to either side of the molding (you may need to undo a couple of side panel screws for access) and then thread them through the web holes to the centre where there is space enough for all the connections to be made and insulated. The buzzer can also go here if it is the small barrel type. This is all done with the molding just next to where it is fitted onto the car, and then rolled over and secured making sure no wires are pinched. Gromets should be fitted at all chaffing points too.
Wiring for the donor wires to the towbar cable should follow the previously mentioned diagram but you will need to DOUBLE CHECK the outputs from your light custer socket. I assumed they would be symmetrical after the first two or so pins were checked, i.e. both outside pins (No. 1 pins) performing the same function, No. 2 pins the same and so on... They were not and goes some way to explaining my confusion in previous posts.
Before plugging the donor plugs into the spare light cluster sockets for the first time, snap off the small spiggots that sit below the sockets. The plugs will not go in without removing this molded feature first.
If you have checked and double checked your wiring connections and are sure all is correct, give 'em a try on a connected light board or caravan or what have you.
It's not difficult, should save you money and gets you more familiar with your vehicles systems.
Good luck