[dsraven]

I guess firstly you will need to clarify whether it is an AD truck or a TF truck. they changed designs mid model year so your truck could be either.
it could be tough finding a good heater for either truck, one that will hold pressure and has a good fan motor, but you may find one in the classified ads here. lots of guys have added a generic after market heater and simply added some spots where the defrost hoses can connect. you may find something on a marine site if you look for a ski boat heater but a lot of those original heaters seem to be bought as a last run from a supplier and you may find the parts are non existant. try a site that sells bus heaters or auxilliary heaters for equipment that runs the correct size hose and the correct voltag for your truck.