Any SPST or SPDT relay will work. A SPST relay will connect the primary circuit when the trigger is live. A SPDT relay will have an output when trigger is low or high and switches when the trigger changes (not connecting the NC output wire from the relay essentially converts it to a SPST). You can get these with sockets off Amazon. I'm pretty sure
@linuxgod ran new wiring from his battery, but you could use the 30amp circuit from the connector as the source and then piggy-back into some other ignition only circuit in the back of the truck as the trigger. Then you wouldn't be relying on voltage sense to be your cutoff.
A SPST relay has 4 connections. The source (pin 30), the trigger (pin 86), a connection to ground to complete the trigger circuit (Pin 85) and then the load or output (Pin 87). Pins 86/85 need very little amperage to work, so small cable can be used and then pins 30/87 need to be sized to carry the load of your circuit.
A SPDT adds one more connection (pin 87a) which is the load or output that is energized when the trigger circuit is not energized.
Here's a quick drawing, as stated above, just remove the 87a pin and that would be a SPST relay.