PerryB

You have a couple options at this point. The "factory" approach would be to get the bed harness for a tow package equipped '08. It plugs in somewhere behind the cab and above the gas tank. The neat thing about this method is you'll end up with a full function 7-way. Reverse lights, electric brake and charging. At this point it'll be just a matter of plugging in the fuse and relay and off you go. If all you need is charging, just run a 10 ga. wire to the battery and install a heavy duty fuse holder with a 20A fuse or circuit breaker (I like breakers). When parked, it's a good idea to unplug the trailer so it can't draw down the truck battery.

PerryB

I'm going to revise my last statement. You MIGHT have an unused plug in the left frame rail near the spare tire that has all the wiring for a 7-way.

old_programmer

There are actually two circuits to wire:

1 - battery > inline circuit breaker (or fuse) > relay load lead in > relay load
lead out > plug at back of truck.

2a - switched side of fuse in fuse box > relay coil

2b - if there are two relay coil connectors like my example above:
switched side of fuse in fuse box > one side of relay coil
remaining side of relay coil > chassis (ground)