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I do the same, solder only (solder will only corrode on the outside, not the inside) the problem is that the solder lasts but, the wire corrodes where it goes into the solder, so like "Guts" said, I use the heat shrink tubing, liquid letric tape, and "Die-lectric" grease where ever I can. I have found that running the wires through "Hose" or "Poly tubing" (that is U.V. Resistant) is easily secured with "Cable ties", my boat is very exposed and I run as much wiring as I can through these types of tubing, it really extend's the life of the wires, I'm pretty sure the problem is "U.V." exposure, that sun beats up anything. You can seal the tubes up but, "sometimes" on a boat it is not a good idea to seal things up (moisture trap), so where I can, I just secure the conduit pointed downward (or a drip loop) to drain if any water gets in. Romex (homey-depot) works very well, I've had an exposed piece on my boat for 20 years, no problem, as mentioned "Check the ground", for some stupid reason it's always a bad "Ground" wire on trailer work.