I'd get a test light and check your fuse panel for your switched accessory power. The Instrument panel lights....same thing. I don't have a wiring diagram of the truck...so I can't just pick out wires for ya. What I did was exactly what I said. It's kind of a pain the rear, but it gets the job done.
I would ASSume the 3500's have identical dashboard wirings as the 2500's. I just did this yesterday so its still "fresh". Behind the headlight switch panel( which simply snaps in & out), you'll find a mess of wires coming off the switch. The one you want is orange w/brown stripe on the bottom row. Watch out cause there's a orange w/black stripe right next to it. Hope that helps, Tom