Trailer wiring help

Partially true. You do NOT want to use a "starter relay." You want to use what is specifically rated and is in fact called a "continuous duty solenoid." An actual Ferd relay won't last long.........perhaps 15 minutes...........before it gets hot and burns up

This is because the coil in a Ford relay is much heavier and draws more current. The reason for this is the current rating on the contacts. "Part of" the engineering behind the current rating on relay / switch contacts is the surface area and the PRESSURE EXERTED on the contacts.

I got some PRESSURE for ya. :D

I used the words "ford type" for the visual part, but thanks for the added info.