Interior keeps steaming up

LEAKING on passenger floor plugged up as in no heat.

Yes. The heater core is leaking bad. The white smoke is the coolant and steam. Be sure to add water to the radiator before you burn up the engine.

if the "steam" leaves an oil film on the glass, your heater core may be leaking coolant.

try cracking a window, or set the heater to pull in outside air, check your heater core for leaks. Heater core being my first thought, as the leaking coolent/steam will dampen the whole interior of the truck.

Sounds like it could likely be a hose that goes to the heater core.

Is it smoke? Or is it steam? If it's really smoke, it will smell like burning wires and you have an electrical short somewhere. You're going to have a fire unless it's fixed.
My guess is that it's probably not smoke or you would have already had a fire.
With that said I'm going to say that it's steam in which case your heater core (which is a small radiator located under the dash) is leaking coolant and the resulting steam is rising through the vents. The steam will have a sort of sweet and yes, a steamy smell to it. It will also leave a funny film from the radiator coolant coating the inside of the windshield.
The heater core should be replaced if my assumptions are correct.

If you feel warm air blowing up on your windshield and If your heater is working, I would guess its a coolant leak, probably the heater core. Usually a coolant leak will leave a kind of "greasy" fog that is more difficult to wipe off. It is basically steam coating your windows. You should be able to smell coolant if its leaking.
If no warm air is blowing, I would guess an issue with your heater control module or a vacuum problem of not supplying vacuum to move the plenum door.

Hope that helps.

to clear the interior of fog you must put the fan on recirc and #3 fan position.with the a/c on HOT.it will clear as soon as any heat from the engine is allowed to the heater matrix to dry out the interior.if you allow fresh air into the car with fogged up glass,they will get worse as the temp is warmer inside the car than out.(they collect condensation).

possible small leak in heatercore , a leak around windows,doorseals,or brought in by boots/clothing which is allowing moisture in

u do have a leak, have the head gasket tested for internal leak into the cylinders.