Well I just finished installing the first of at least three headlight relay harnesses into my vehicles. The F100 got the first one this morning. Decided to make my own so I could control the quality of the parts. Used genuine Tyco/Bosch 12V 40/30 amp SPDT relays, a Bussman self reset fuses, found high quality 14 ga. headlight sockets with crimp and solder terminals and all other wire 12 ga. primary in different colors.
Actually simple. Our older vehicles route all power for the head lights thru the switch in the dash. That is a fair amount of power in an older switch and wiring. Similar to how our ammeter is wired behind the dash panel.
Anyway, by the time the power goes through the switch and gets to the lamps your 12V could be down around 9V now. That also means the lights will be less bright. To get power away from the switch and to supply a full 12V to the lights, or support high draw halogens, then relays are the way to go as in cars of today. I pull power off the battery side of the solenoid to the fuse and from the fuse to each relay. One handles low and the other high. You activate the relays by using a trigger wire from your original head light socket to the relays. This turns your light switch into just that... a simple switch only and now brighter lights to boot.
I used this guys design here. He does a really nice job with plenty of photos.
I did the relay upgrade about 7 years ago and upgraded to H5 bulbs and they are nice and bright now. I found a wire diagram and instructions on this web site somewhere. I have since found a guy who has the connector for the relay so you do not have to terminate directly to the relay makes it easy for relay replacement. two relays two bases with pigtail two headlight plugs with pigtails for $15.00.
I need to remove the old stuff and convert to the new system.
I saw the socket that would fit these relays. Problem was that none of them used 12 ga. wire and therefore they were of no use to me. As for my relays each one cost $12 since they are Tyco/Bosch and not Bosch style. The head light sockets were $10 ea. as I wanted a minimum of 14 ga. and most that I saw locally were 16-18 ga. Could also get weatherproof 12 ga. soclets for $12.
This is why I didn't bother with packaged assemblies as their wire gauge was smaller than 14 ga. in many. Even my power wire is 10 ga. since there is safety in thicker wire especially if you run H4 with their power consumption and heat created in wire.