Helpful hints on JDM Headlight wiring by Train
Originally posted by Train:
1. First off, you CANNOT use H4 bulbs with the one-piecers unless you modify the base first. The base of the H4H is a lot smaller than that of the H4, so you'll have to trim the H4 to get it to work. On top of that, the filaments in the H4 are located much closer to the base than in the H4H bulb. What this means is that your beam pattern will be off significantly.
2. If your one-piecers didn't come with harnesses, you can just use a harness from an H4 bulb that is easily obtainable at your local auto parts store. Although the bases are different, the plug configuration is the same between the two bulbs, so the H4 will work. It is not possible to use the stock 9006/5 harnesses with the one-piecers unless you do some major frankenstein work as some have done here already. You can modify a 9006 bulb and use your stock harnesses, but again, this involves modifying the base of the bulb to match the H4H so it is seated properly in the housing.
3. Depending on what clip is broken, it should not adversely affect the stability of the housing. I've found that the 2 bolts that attach at the top of the housing do the most to secure the housing in place. As long as these ones are intact and most of the other clips are not broken, you should be fine.
4. Now for the big question that you all have been asking...drum roll please! How does one wire the lights? There are several different ways to do this. The easiest way which I think a lot of people have done and to which drinkdontdrive alluded earlier is to wire your low beams to the H4H bulb and your highs to the H3 fog. The stock harness consists of 4 wires. You need to properly identify the positive and ground wires for each of the low and high beams and also find which wires coming from the H4H harness are for the low beam and the ground. Simply splice these wires into the proper ones on the stock harness. Do the same for the fogs but splice them into the high beams. What you end up with are regular working low beams, but when you flick on your high beams, your fogs will come on as well as your high beam light.
Now the correct way to wire the lights would be to run both the low and high beams to the H4H bulb and wire the H3 fog directly to the battery via a switch going into the passenger cabin. The problem with the stock setup is that the high beam wire is always hot unless it is properly grounded which runs through a relay. The H4H harness consists of 3 wires: low beam, high beam, and a single ground. If you try wiring the stock low and high beams directly, you'll end up with both beams lighting up at the same time. What you need to do is run them through a relay so that when you turn on your lows, the circuit for the high beams is cut and when you flick on your highs, it cuts power to the low beams. This way you'll have functional low and high beams as well as fogs if you choose to wire those.
Hope this helps everyone.