If it's not blowing out hot air, the blend door may be the problem on the 1990 Isuzu Trooper.
Heater Water Control Valve
REMOVAL & INSTALLATION
On early models the heater control valve was part of the heater core inlet. To replace the control valve, remove the heater core as outlined in this section. Remove the control valve from the heater core, being careful not to damage the core. Use new gaskets when installing new valves.
The remainder of the vehicles are not equipped with heater water control valves. They use heater unit doors to regulate the amount of heat entering the vehicle.
I would check that both tubes running to the heater core are getting hot. If they are, then there is a blend door problem. If both tubes are not getting hot, the heater core is blocked somewhere or the heater control valve, if so equipped, is in need of replacement.
Good luck on this repair.