When incandescent bulbs are fed more voltage, they consume more current too, so the engine running having the voltage up in the mid 14's is enough to blow 20 amp fuses but not 25.
But there is still obviously a problem somewhere as these bulbs should not be blowing a 20 amp fuse.
About the trailer wiring harness, the actual connector itself is less likely the issue compared to where the installer took the power from.
They use these wire taps which one places over the wire, and squeezed to puncture the insulation and make contact with the wire.
These will always fail at some point, it is just a matter of time, because the insulation is compromised, and the very act of tapping the wire here breaks a lot of the wire stranding.
So I would investigate the trailer wiring harness where it taps into the original wiring harness. The headlamp switch itself on the dash passes all the current for the signal lights, and the headlamps too. Look for burnt insulation or corrosion at the headlamp switch connector.
get a can of this:
http://www.amazon.com/CAIG-DeOxit-C...28987560&sr=8-1&keywords=CAIG+d5No other electrical contact cleaner comes close. others just flush dirt and grease, Deoxit removes oxidation, and enhances conductivity of the mating metals.
Do note that Di-electric grease prevents water and oxygen intrusion and thus corrosion, in theory. It also helps heat transfer, it does not clean electrical contacts and does not conduct electricity.
When it gets really old it can actually cause issues.