Hi Jim,
Although you didn't say for what truck, the easiest source of boost signal is the large plugged hole just forward of the MAPP sensor on the left (driver's) side of the engine on a 24V. It's a 3/4" NPT (I think) plug - remove it, replace with a fitting drilled and tapped for 1/8" pipe to connect your gauge line.
I took gauge lighting from the tan colored wire on the back of the headlight switch by just connecting the gauge pos wire to it. Makes the cluster dimmer work with the switch like the rest.
I think that it's also the #14 5a fuse in the box on the leftside of the dash, so you could go there too.