I am fixing up a trailer to pull behind my truck. Is it OK to use standed 18 gauge speaker wire to wire up the brake and stop lights? I went to Radio Shack and all they had was speaker wire. If anything it seems a bit stiffer and thicker than what's on the trailer now.
Thanks in advance.

You can - but how long will speaker wire last if used or left outside in the Winter? Don't you think for something as important as a turn or brake light you ought to do it right?

Agreed.

Speaker wires are mostly PVC insulated jacket. I think they are rated for 50-100v which should be no problem on a 12v system. However the jacket are not listed the same as regular electrical wire. Wire have listing such as THHN or THWN. Thermoplastic, high heat or high water and nylon coated. Their properties give the insulation more weather, oil, solvent, gasoline resistant. So aftersome outdoor beating, speakerwire may start to deteriorate and have insulator crack.

Instead of Radioshack, is there a Homedepot, Orchard supply or Lowes nearby? They will have what you need.

Yep, or an auto parts store.

K' Thanks.

Do you trust crimp connectors, or would you use solder - then insulate with tape or shrink tube?

I hardly have problems with crimp connectors. Use the right crimping tool. Some crimping tool are made for non insulated buttsplice and will cut into the insulator of the splice. For a better look, you can stagger the point of splice so that you won't get a big bulge in one spot.

Sometime the insulator of the wire can get stretch out far enough that it is out of the sleeve of the crimp and the wire will be expose. Usually I seen this happen when the electrician strip back the insulator to far back and not trim the copper wire resulting in the wire insulator not insert fully into the sleeve or if the insulator of the wire was pulled. You can alway use a shrink tube to slip over the splice and that will help prevent corrosion and a better look. Before heating the shrink tube, you can squirt some clear silicone into the shrink tube then apply heat to shrink it. The excess silicone will get push out during shrinking. This will make it waterproof.

Soldering is more a secure way of joining small wire assuming you get the wire hot enough for a good wet flow of solder and not a cold joint. Becareful when you remove the soldering iron. Sometime the solder will follow the direction of the iron and create a sharp point at the splice. That point can cut through a shrink tube and create a short.

They have a wiring kit for trailer, proper color codes. For long term
I would go that way. Solder and heat shrink. If you live where they
treat the roads in the winter, any exposed wire will corrode quickly.

The speaker wire is not made to flex. The cover is not made to handle
heat.

Any auto parts store will have what you need, and in the long run it
will save you time and money and be much safer for you and others.

https://www.harborfreight.com/four-wa...kit-96658.html

Another point, if you have a later model car, 2000 and up, you need to get a harness
for the car as well. Cutting and splicing in the cars harness can cause major problems
with the vehicle. The one you need is made for your car and no cutting is needed, just
plug it in per instructions.

Some cars come from the factory with the wiring in place. On ours it was hidden under
the trim panel in the trunk.

Besides an auto parts store, try an RV dealer for a wiring kit.

Walmart too.