Here's what I did recently. I ran a #12 ga. brake cable along one side of the frame and tie-wrapped it to the propane pipe. I ran the new brake cable across each axle tube and secured it with tie-wraps every 6". The cable is on the trailing side of the axles and not likely to be hit by road debris.
I soldered all the splices, used shrink wrap and then electrical tape over that for more physical protection. The splices are well-secured to the U-bolts with tie-wraps. The brake wiring installed by the axle manufacturer is tiny #18 ga. (in our TT anyway) and the holes into the axle tubes are waaay too small to get a #12 ga. brake cable into. The alternative is to run new cable above the coroplast but that's a heck of a lot of work for little gain, plus the wiring will be a bit longer.
I bright the brake cable up from the backing plates and forward about 1.5' to 2' forward and tie-wrapped it to the propane pipe. This allows for a lot more vertical movement than will ever occur under normal road travel or jacking the frame up to re & re a wheel.
I have to say the improvement in braking with the new #12 wiring is absolutely amazing. The original splices by the TT manufacturer were really sh!tty - not only added to a poor connection but there was the possibility of the splices pulling apart. See the photo below for one example. The crimp connectors are designed/approved for one wire only in each end and note how only a few strands were pushed into one end. ALL of the other splices were equally bad.
