I think your GM truck has a fairly large fuse box mounted under the hood. You just need to find one of the fuses that is live with the ignition on, and tap into that source.
You can use a Fuse-Tap device to connect it to the fuse itself and run a wire to activate the relay. This is what I'm talking about, and its sold by Bussman. I found it at one of the auto parts stores - maybe Pep Boys.
It would be better to find the actual wire to tap into instead of the fuse box, but without a color-coded wiring diagram that might be tough to do.