I haven't done the relay thing to my truck yet, I've done it to almost every other car I've owned. But it is going to get done the the truck. I plan on posting better info when I'm done so you'll know where the easiest places are to splice into.
Newer trucks have ground side switching, and that's what I'll explain. The goal is to reduce voltage drop in the h-light circuit to allow more voltage to be available to the bulb for making light. The high beam circuit is RD/OR and the low beam circuit is VT/WT.
I go get ISO relays from the salvage yard with the connectors and some wire off the connector, you'll need two relays, one for the low beam circuit and one for the high beam circuit. Remember; a relay has two circuits A. High current circuit that is controlled by the contacts. B. Low current current circuit that goes through the coil in the relay to turn it off and on.
The procedure basically goes as this.
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Using a typical ISO relay make the following four connections for each relay:
1. Take the existing 12V wire from the light bulb and cut. Wire the end from the light bulb to pin 30 on the relay. This will be the feed for the contacts in the relay.
2. Take the other end of the wire you just cut, that goes to the light switch, and connect it to pin 85; this will provide a ground for the control coil of the relay.
3. Add a wire from pin 87 to the battery negative terminal, a nice short path to ground. This is ground for the contacts. No need for a fuse as the bulb feeds are already fused.
4. Add a wire from a B+ power source (only flows about 500mA) to pin 86. Fuse installation is up to you on this one, not a bad idea though. This is the 12V feed for the control coil.
There you have it. The headlight switch and dimmer switch only have minimal current flow. The lights are brighter because almost all availble voltage is available to the bulb filament, instead of 1 volt or more being "dropped" in the harness.
The OE splice (S106)for joining the left and right high beam circuits together are in the area of the underhood relay center. The low beam splice is in some kind of "joint connector", I have no idea what that is. That's the areas where I intend to cut in and install my relays. There may be a better place but I'm not aware of it yet. I'll post my findings when I the the job done.
HTH, if you have more questions please ask, I don't check the board daily so there may be a delay.
Wayne