Not sure why you'd rather rig a push button versus the auto heaters, but that's your choice. I'd use a momentary push button, that way it cant be left in the on position, and burn something up.
Google "push button grid heaters, Cummins diesel" lots of good info, what to use, and even pic-to-grams of what, how, and where to wire it, the proper wire and fuses to use to keep you from turning just another grandpas truck into a smoldering pile of scrap metal
Hope this was helpful enough, otherwise the experts on facebook will surly point you in the right direction
'93 W350 5spd Getrag, 4.10 gears, 83k miles and counting, Mods: LPPP AKA Ma Dually
'54 Dodge M37 6.2/700R4/205, 5.89 gears, WC rear axle