DIODES AND TOAD LAMP WIRING
The drawing above shows the preferred method of installing diodes in series with the rear Turn Signal/Brake
lamps on a towed vehicle. I prefer to use two diodes for the following reasons:
There is always a possibility that there is some type of delicate sensor or computer input connected to the Brake and/or Turn Signal line in your automobile. A diode between it and your turn signal bulb will prevent the 12 VDC from yor Motorhome from entering that path.
Your Signal Flasher in your automobile is typically rated to handle 4 to 6 bulbs. Unless you place a second diode between the tail lamp and your motor home feed, you can accidentally overload your auto flasher system by attempting to illuminate all of your motor home bulbs. Granted, you do not normally use your turn signals on your car while interfaced but Murphy's law says that if it can happen, it will happen. Another even worse case situation will exist if you accidentally trigger your automobile Emergency Flasher system while interfaced. Without the second diodes on each bulb, I can almost guarentee that this will cook a 6 bulb flasher since it will be attempting to handle 12 or more lamps.
Above is a simplified drawing of the preferred Park and Running Light interface. Since all of these lamps are connected in parallel there is no need to run a line to the back of your vehicle to interface with this circuit. I typically connect into one of the Front park lamps that is closest to your interface connector. The diode is a matter of personal preference since there is a possibility of damage to your Park Lamp Switch if you plug your interface in with automobile Park Lights on. In this case said switch will be attempting to illuminate not only your automobile lamps but also all of your Motorhome lamps. Motorhomes typically have a "LOT" of lamps. Typically, it will just blow a fuse in your automobile lamp circuit.
CONNECTOR MOUNTING BRACKET FABRICATION
The Truss Plate in above photo is what I always use to fabricate my mounting brackets for the interface connector's. It is about 3" wide by 7" long and has a number of desirable features for this application.
To make your bracket, you will need a holesaw large enough to cut a circle in it for your connector, a piece of scrap 2 by 4 lumber, and a pair of tin snips. Try to determine in advance what size and number of holes you will need. Then temporarily nail the plate to the side of the 2 by 4 so you can use it as a holding fixture while you are drilling your holes. If you need to bend it, simply mark your bend line and place it over one edge of the 2 by 4, nail one end and bend it around the edge with a hammer. Use the tin snips to cut it to the desired size and to cut off the sharp corners on finished bracket.
The photo below is my Saturn setup where I made two of these brackets. the one in the foreground accepts the Motorhome harness. The one in the background allows me to remove the Stowmaster towing extensions when I don't plan to tow for some period of time
This is a photo of the Right Rear interface on my Saturn.
The Blue wire connected to the right side of the diode assembly feeds the Right Turn Signal Lamp.
The Heavy Red Wire connected on the left side of the diode assembly comes from my motorhome interface connector.
There is another Blue wire connected to the Diode assembly immediately below the Red wire which comes from the Automobile Turn Signal System. It is hidden in this photo by some of the other standard wiring.
This particular diode is distributed by Roadmaster but they are available from most camping supply outlets. If you are lucky, there will be enough room in your tail lamp housing to handle the additional diode and wiring interface.
