Where exactly on top of the dash? Like, on
top top, where the red and blue arrows are?
I can say that there's a decent amount of room underneath the dash cover in the area of the blue arrow, but it's also directly above the HVAC vents (not a big deal, just keep your wiring away if you're worried about the temperature). Based on the amount of my dash that I've had taken apart (not much more than what you see in the pic) I can say that there isn't a ton of room underneath the dash cover in the area of the red arrow, but there is enough to run a couple wires.
I'd say it would be easiest to do if you could remove the entire upper section of the dash cover, as there's just not a whole ton of room to stick your hands/tools when the cover is still in the truck. I ran the wiring for my overhead lights to the front section of the dash, to the right of the cigarette lighter plug, and there was plenty of room back there to run lots of wires. The only things you'll really need to avoid interfering with are the HVAC vents/controls and the passenger-side air bag, which takes up a big chunk of real estate on that side...which I assume is fine, 'cause who wants to look at a gauge/reach for a switch that's all the way over there?
