Going to have to be a bit more specific, IE: Are to towing a trailer with the car, or towing a trailer with the car in it?

I'm doing Dragweek this year (1972 Demon) and I'm wanting to isolate the car wiring from the trailer wiring. I once heard it could be done with relays. If anyone has any suggestions, tech articles etc. I'd appreciate the help.

I'm not understanding the need for the relays

One is still going to have to use a plug to disconnect the trailer from the car unless I'm missing something.

I also don't understand the need for a #4 to 6 gauge wire

Unless your trailer looks like the house in Christmas Vacation I'm totally baffled.

Sounds like he doesn't want to run the current for the trailer lights through the wiring in the truck?

Having the 4 wires to the trailer triggering their own relays would work. Though it would be overkill, unless he has a few kw of lights.

I have a small fuse block inline with the wiring to the trailer. If a wire on the trailer shorts out, it will hopefully blow that fuse, keeping the lights on the truck functional.

I'm not understanding the need for the relays

One is still going to have to use a plug to disconnect the trailer from the car unless I'm missing something.

I also don't understand the need for a #4 to 6 gauge wire

Unless your trailer looks like the house in Christmas Vacation I'm totally baffled.

Sounds like he doesn't want to run the current for the trailer lights through the wiring in the truck?

Having the 4 wires to the trailer triggering their own relays would work. Though it would be overkill, unless he has a few kw of lights.

I have a small fuse block inline with the wiring to the trailer. If a wire on the trailer shorts out, it will hopefully blow that fuse, keeping the lights on the truck functional.

So he is talking about the small support trailers the drag week cars tow from track to track. A fused power feed directly from the battery. Then a relay for tail lights, two for brake lights/directionals. Tap into the rear tail light wiring harness to get each “signal” voltage for each relay. Only a small voltage will go through the car wiring. Tap into the driver side brake light wire and send it to drivers brake light relay. This relay output goes to the driver side trailer lights. Do the same thing for the separate passenger side brake lights relay. Then tap into one tail light wire on the car and run it to the tail light relay. The output from the tail light relay will spit and go to BOTH trailer lights. Drivers and passengers sides. Three relays and three splices into the car tail light harness.

I also did that but remember the turn and brake filament are the same.

I also did that but remember the turn and brake filament are the same.

just went through a similar situation with my buddy's 54 chevy custom.
it is using 6, 59 cadillac bullet tail lights, [three on each side, frenched into the fenders.]
as he is using the original 54 turn signal switch and wires, he added relays at the tail lights because he figured the multiple tail lights would be an excessive load on the original turn signal and brake light switches.
adding a relay to each bank of rear lights, every imaginal wiring combination of the relays would not make the rear lights function as they should. he would get brake lights on one side only, turn signals that would work one side or the other, but not both, or act as 4 ways, and other goofy combinations of both side rear lights.
i investigated his wiring, and it "seemed" the relays were wired correctly, and i even tried a couple of different ways to wire the relays, including using the ground to activate them. nothing worked. these were good quality bosch OEM relays.
sometimes, the relays would just "hum", and the lights would do nothing.
so just for grins, a pair of 1157 sockets were wired into the rear light harness in series with the caddy tail lights, and now the lights work as they should !
it seems there wasn't enough draw through the relays to trigger the flashers used in the turn signal and 4 way circuits through the original turn signal switch.
adding the extra 1157 bulbs in the tail light circuit[s] brought enough draw to allow the turn signals to work correctly, as well as the brake lights. [even though conventional wisdom would say using the three 1157 bulbs on each rear light circuit should be enough draw to trigger the flasher]
i have never ran into this before, but am tossing this out there for others to learn from, if having troubles in this area.
there are no issues with excess heat in either the tail light wiring, or the relay wiring.
he is going to mount these extra bulbs and sockets behind the upholstery side panels in the trunk, close to the 1/4 panels on the rear of the wheel tubs.
i couldn't believe how this problem needed to be fixed, but i wrote it down for future use.

No need to piece this together.

Tekonsha trailer wiring

I use the direct fit version of this box (plugs into the existing light harness without the sensing feature of this universal one) on my daily, works great.

Check the steering column wiring harness and that switch

Check the steering column wiring harness and that switch

Fun fact if you wiggle the hazard switch that hasn’t been used in who knows how long the rear hazards will work.