Could be the fan switch, the fuse, faulty wiring or wiring connections or the fan motor is burned out.
I own a 1994 Isuzu Trooper, the blower moter is located inside the truck under the glove box.
there is a resistor that controls the speed and should be near the blower motor its called blower motor resistor
I not sure
it could be an alternator problem,is it an automatic type heater(no manual levers)
The 6VE1 3.5L V6 fitted to the Isuzu Trooper was manufactured in-house by Isuzu themselves
Defective switch, resistor pack, or blower motor.
In the 1999 Trooper the relay is sbove the inside fuses. It is above the fuse door, so that the whole panel by bottom of the driver door must be pulled. Don't know if that helps for 1990. Note: My fuse, relay and motor are good, but no power to motor. added 1/22/2008 The blower motor resistor is located on the housing located on the firewall. This housing is were the blower motor is.
The thermostat on a 1998 Isuzu Trooper is located on the front of the motor. It is in the top radiator hose where it connects to the motor. To access the thermostat, the hose needs to be disconnected from the motor.
The 1995 Isuzu Trooper can equipped with a 1.8 litre or a 2.2 litre engine. These are the only engines that will fit in the 1995 Isuzu Trooper. There are motor mount conversion kits that will allow for other engines
The starter for the 1992 Isuzu Trooper was a little difficult to locate, honestly. One may find it on the driver's side of the motor, directly above the oil pan.
The Honda Motor Company is not a relationship with the Isuzu vehicles.
getting ready to put the motor from my 1989 Trooper into a 1986 Trooper II... will let you know how it goes!