Heater not getting hot - 1996 Land Rover Range Rover

Turn the heater temp control to high, then other side will get hot as water flows thru heater.

wires get hot through either a bad connection or too much power being drawn through them, as for the heater blowing cold is there any hot water flowing through the heater check the pipes for the heater that pass through the bulk head

Sounds like the heater core is blocked and not circulating hot water. Uncover the heater core under the dash, little radiator. Get the vehicle to running temperature, turn the heater on and feel if the little radiator gets hot. If not then something is blocking the water circulation. Either the heater core is broken or the lines are plugged coming in or going out from it. Typically it is the core that is broken.

If the heater core is getting hot, you could have a problem with a blend door under the dash or the HVAC controller which opens and closes the doors.

Either your thermostat is broken and jammed open (this means the water in the heater matrix will take forever to get hot) or you have an air-lock in the heater matrix. To check the thermostat, the engine should get too hot to touch before the radiator gets even warm. If they all get warm together, your thermostat is faulty. To bleed the heater matrix, uncouple the higher of the two hoses from the bulkhead an top up the water resevoir until you get flow from the loosened heater hose.

Possibly the heater core is 90% blocked. open enough to draw hot water in, (hose feels hot) but not enough volume to provide heat from very cold air, 10% hot water gets cold quick, coming from core..
Flush heater core in reverse direction - cold side in hot side out..

Just a theory..

Do you know how to carefully feel heater hoses to see if coolant getting to heater core is hot enough? Has thermostat been changed lately, and coolant system flushed?

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.

check the heater temp. control pot under the hood, inline on one of your heater hoses, it may be sticking causing hot water to only get to your heater core when the engine is at high r.p.m.s

heat exchanger not working look under dash find where cable goes from heater controls the hot and cold switch move back and forth to find it its not opening all the way when you move it to hot push it farther by hand this will open the airway for your heater core