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.

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 towing a trailer with the Demon during Dragweek. I dont want to burn down my stock wiring nor do I want to have issues with the switch.

Why not use the basic four wire trailer plug set up? Tail lights,turn signals and brake lights.

I've used 25 amp relays in several different uses and applications, you can to
You can use the OEM wires to activate the relays (battery input to the on side of the relays) and use a bigger battery feed wire to all four relays, each light circuit or individual battery feeds to each relay
it is your choice on which way to go, None of the light circuits should have a large current draw so using a #4 up to a #8 gauge battery feed size wire to feed all the relays individually should work well
I would mount the relays in the trunk next to the driver's side taillights on a homemade bracket and tap into the trunk wiring harness there to do this job I would remove around one inch of the wire insulation, don't cut them apart, and then wrap the new wire pigtail going to each relay and solder and heat shrink insulate each joint

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.

I agree! Trailer description please. If there are no brakes on the trailer a simple 4 wire plug(wired correctly) and good trailer grounds....done.

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.

Maybe I am reading it wrong?
I read he had a 1972 Dodge Demon, and wants to tow a small dealer with Demon not truck.
His plan I assume is to, pull a small trailer with extra parts, tools and supplies. Small trailer with stuff in case something needed in route.
I believe the rules state you must be stand alone vs a chase team to repair
Guys toss slicks, drag radial on roof, back seat etc. So they have highway tires on car ect. And how small trailer with tools, parts, few race gas cans ect.
He sound be fine with a 4 pin for lights, signals, brake lights on a tiny trailer. (ASSUME) He is towing with an A Body

Read his post again , he wants to isolate the trailer lights from his stock wiring system with relays

same reason and concept putting headlights on a stand alone relay setup, load off the the marginally adequate stock harness.

Use LED lights for the trailer, minimal current draw, no need for relays.
As mentioned, add a fuse to protect the car wiring, if anything shorts on the trailer.

lots of ways to skin the cat. pick your parts, pay your money. Any additional load on the stock 72 electrical system isn't the way I would go. JMO

Work the approach that suits you best.

I use relays, a fuse block and circuit breaker in the manner that Cab describes. But, I tow bigger trailers and occasionally not mine trailers.

The not mine ones are usually sketchy and I got tired of losing my truck lights when plugging in.

An isolated system is easy to install. I put my stuff near the battery to limit the length of high Amp wires to the fuse block.

And yes, the factory wires are taxed by extra lights.

Thank you 340six, Cab_Burge, ruderunner and crackedback for reading the entire post and answering or giving input related to the question. I appreciate it more than you guys know. I figured the system Cab described was what I needed but didn't want to presume it.

Cab_Burge do I need to run a wire from the switch all the way back or will just tapping the light wires serve as the trigger? Just ensuring I'm correct that I will only use 3 of the pins on each relay for this project.

Crackedback was it you that built or builds the headlight isolation harnesses? If so is it needed if I'm running LED headlights?

Thank you too for the input, I am not one to rely on "hopefully" although a standalone fuse panel was considered.

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 think this works.

Thanks FurryStump, that's a simpler setup than I had designed. I had the brakes and turns on separate relays.

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