Trailer wiring help

Cole-Hersee makes continuous duty solenoids. They're a three wire hook-up; power in, power out, and ignition source.

The ACR's or VSR's are also a three wire hook-up; power in, power out, and ground. I like these better than a continuous duty solenoid because they're smarter about when they make and break the connection (allows the starting battery to be recharged before connecting, disconnects automatically when the alt stops charging), but they do cost a bit more than the solenoids do.
$96 http://www.delcity.net/store/Voltage-Sensing-Relay-!-Single-Charging-System/p_793852
vs.
$25 http://www.delcity.net/store/Solenoid-Switch-!-100A-@-12V-Grounded-Continuous/p_790161