If you pull your fuse box under the dash, behind it there is a group of four orange wires with various colored stripes, these are the leads from you upfitter switches, the 2 smaller ones are the 10 amp fused, and the larger are the 30 amp fused. Sorry I'm not sure on the stripe colors.

Someone here I'm sure will be able to tell you.
Once you choose the upfitter switch (orange wire with stripe) just connect that wire to the positive wire on the guage for the backlight, and ground out the ground, flip the switch and it should work like a charm.
