# VW MK4 OEM HID - Relay harnesses?



## vwtuner4ever (Feb 17, 2004)

Hello,

I am wondering why the OEM HID are plug and play, while any type of retrofit needs relay harnesses. It just doesn't make sense to me. Can anyone explain how come these headlights don't need harnesses? 

Can such set up be done on retrofits to avoid using harnesses?

Thanks


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## dennisgli (May 17, 2003)

I believe that you just need a 10-pin to 12-pin adapter cable - they are not plug and play - and they do not need a relay harness. The OEM halogen headlights have a 10-pin connector and the OEM HIDs have a 12-pin connector.

Golf/Jetta IV Headlight

Golf/Jetta IV HIDs


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## wankel7 (Nov 30, 2013)

vwtuner4ever said:


> Hello,
> 
> I am wondering why the OEM HID are plug and play, while any type of retrofit needs relay harnesses. It just doesn't make sense to me. Can anyone explain how come these headlights don't need harnesses?
> 
> ...


I just finished up a retrofit on my MK4 Jetta.

http://forums.vwvortex.com/showthread.php?7160582-Jetta-TDI-HID-Retrofit&highlight=hid

From what I can tell you do not NEED to use a relay harness to do a retrofit. However, it is a good idea to use one and considering how TRS charges only $35 I do not understand why you would not use one. The quality and weatherproofing on the harness is top notch and I really think their profit isn't much on the harnesses. However, they are really key to making a retrofit's wiring go smooth so maybe that is why they are inexpensive?

The TRS relay harness plays very well with the solenoids that move the shield so you can have high beams. 

As far as the OEM wiring I really don't get it. One of the features of OEM ballasts is their near instant start up time. They accomplish this by a very high initial wattage that is sent to the bulbs. Normally , this means the wiring needs to be beefier so since the OEM hids are plug and play I don't get that part. Maybe ALL models have wiring that can support the initial load of HID ballasts? However, if your retrofit is using an OEM ballast a relay harness becomes more important since the TRS relay harness is tied straight to the battery and uses two ground wires. 

Wiring, running, and installing the retrofit harness was no big deal and you should consider just using it.


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## vwtuner4ever (Feb 17, 2004)

Yes i know and i actually do have harnesses. I was just curious. I still find it odd.... its one of these mysteries in life you know :laugh:


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