There should be a small radiator inside the cab, burried in the ducts, the fan blows air thru that radiator to generate heat for the cab. If your heater isn't working, but the fan is, then it can be either a plugged heater hose, clogged heater core, or ducting that isn't opening to allow the heat out. First thing I'd check is to make sure there are 2 hoses from the water pump to the firewall. Those are your heater lines. They plug into the heater core. My (in the dark) guess is the PO bypassed those lines.