as chryco said, any parts store. What you will get is a little black box with four terminals on the bottom. Each numbered, 30, 87, 85, and 86. (maybe 87a which is a resting point for the relay's switch, directly connected to the #30 terminal)
Basically Terminal #30 is your '12v in', should be fused and can be off of battery voltage (always hot). Terminal #87 is your '12v out' which is what you are powering (your line lock red wire, like lock should be grounded to the chassis)
Terminals 85 and 86, one of them needs to be grounded and the other needs power (not much). I usually use a 'ground switch' for relays, since its easier to find grounds than it is to wire up a special power wire for the switch you are going to use. So lets say for your line lock, you have a little red button or something which is common. Should have two wires i assume? Go ahead and tap into the #30 wire, and give terminal #85 a 'constant hot'. Then take a black wire from terminal #86 to one of the wires on your red button. The other wire on your red button to a ground. What you then have is a ground switch, with very little current going through your actual switch. Tap the button, it sends a ground signal to terminal #86, #85 has power. The little switch in the relay gets pulled over (pulled over? the 85/86 terminals inside the relay have a coil in there with a LOT of SMALL winded wire, power and ground to the coil creates a magnetic field pulling the switch in the relay over to power what you need. Pretty cool, is also what makes a relay effective, letting small current, control big[ger] current) to touch terminals #30 and #87 and you then have power to your line lock.
Make sense? lol its early.
