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Wiring a 2007 VW Eos 3.2 with Multiplexed Wiring for Towing a Trailer

Question:

Hi, I have a 2007 VW Eos 3.2 and want to get the electrical wiring for the trailer lights done. The VW dealer says he tried to install my Modulite heavy duty circuit protected trailer light power module but that the computer detects it as a problem and as a result none of the lights are working. so they disconnected it again. Any experience? advise? How can I get the trailer lights connected?

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Expert Reply:

The 2007 Volkswagen Eos has a Multiplexed wiring system that requires wiring the Upgraded Heavy Duty Modulite Circuit Protected Vehicle Wiring Harness, item # 119190, for a trailer connector. There are 3 ways of wiring depending on how your vehicle is wired from the factory. In the instructions, at step 9 (see link), it explains that you may have a two wire system that requires the red stop wire to be grounded along with the white wire. If you have the 3 wire system (most likely) with amber turn signals or separate red bulbs for stop and turn signals, you will need to do one of two things. If you have an independent tail light circuit then you will need to attach the red wire to the combined stop and tail light circuit and attach the brown wire to the independent tail light circuit. If the stop and tail light functions are on the same circuit, and there is no independent tail light circuit, you will need to attach the red wire from the Modulite to the stop tail light wire and attach the brown wire to ground along with the white wire. When these connections are made you will need to run the power wire to the vehicle battery.

The system on your vehicle sometimes causes a bit of bleed over when the trailer is connected. This will happen because of the combined circuits and is not a problem. I only mention it because sometimes the bleed over can be detected at night and it tends to be interpreted as a ground issue, but is not. You trailer lights will work normally and will be bright like they should be. When there is a bad ground the tail lights, brake lights and/or turn signals will be dim.

It takes a bit of testing to figure out which system you have but it is not that difficult once the instructions are understood, and you have the correct wiring harness. Once the harness is wired correctly your computer will not detect a fault.

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