Originally Posted by Dirtman

Ok first your gonna wanna rule out the very common issue of just having a crap temperature sending unit giving you false readings. Remove the sending unit and hook up a multi meter to it on ohm setting. Attach the ground to the body of the sender and positive to the terminal. See what it reads cold, then dip it in a pot of boiling water and see if that number changes. (The exact numbers aren't that important, mainly because I don't know them off hand lol) But if it doesnt change much or at all you need a new sending unit and the heater is a separate issue. If it works, re install it and test it again with the multimeter while on the truck after its "warm" wait 15 minutes or so with it idling. If the readings are the same as during your boiling pot test, your gauge or wiring is broke. If it now reads cold on the multimeter you know it's an actual problem with the engine not warming up.