How do you change heater core 1990 Isuzu pick up?

You've picked an elaborate repair, way more elaborate than it should be. Make sure the core is definitely the problem - you won't want to do this if you don't have to. Remove: steering wheel, all plastic steering column trim, instrument cluster and housing ( speedometer cable is snapped to the back, no tool required just small hands). Remove lower dash panels, some misc heater ducting under the dash, radio, radio mounting bracket, and the dashboard support bracket behind it. Disconnect 2 control cables running to the white plastic heater box (now visible) and 1 running to the air source under the right hand dash. Disconnect cigarette lighter wiring. Disconnect fuse box from dash assembly (drivers side bottom). Remove dashboard - it's held at five more places: the bottom two corners, under two vents at the top dashboard corners (they pull out) and under a plastic cover center top of dash (must be pried out - try not to damage the plastic). Drain engine coolant. Disconnect the two heater hoses from the heater core pipes where they protrude into the engine compartment through the firewall. Best to replace these hoses now, while they're accessible. They're cheap. You can now disconnect the plastic heater box assembly from the firewall and remove it from the vehicle. Remove all of the brass screws holding the box halves together and it will come apart easily. The heater core nests inside, held snug in the box by foam seals. These will often be hardened or disintegrating and if so should be replaced; otherwise air leakage around the core will cause poor heating in winter and heating of the cab in summer. Note: unlike most other cars this Isuzu has no valve in the coolant hose. In this terrible design the heater core is constantly hot and there's no easy way to shut off the fluid flow if the core develops a leak. Many owners will be forced into an immediate and unreasonably expensive dealer repair, which is the whole idea behind this. Assembly is the reverse. Good luck. The only reason you would be reading this post is if you are faced with the task of replacing the heater core. I'll skip the usual points of disconnecting battery, draining as much coolant as possible - I checked the Hayes Manual - their solution was the usual quick sweet way which never seems to work and I did consider the above task as well. Looking for an easier way out this is how I re and re'd the heater core on my 1992 Isuzu P/U Extended Cab without airconditioning. I hope it saves you some stress. The task is not easy, about 4 hrs inside the cab. So we begin: Remove the glove box (easy), remove the lower trim under the glove box (easy), remove trim panel over the radio (easiest), remove the ashtray and ashtray housing (easy), remove the front metal housing the radio slides into (a little struggle), here's the key to removing the heater box through the right side which makes life a whole lot simplier, remove the metal dash support directly behind the metal housing you just removed (it is what prevents the heater box from being pulled out to the right), there are two 10 mm nuts into the floor and two on the top of the bracket (medium) - once you have it out cut off about 1/2 in from the top metal tab (makes it putting it back in easier. Now back to the heater box itself - unplug the power on the heater box, squeeze the accordian box together and pull it out (easy). Unsnap the heater control cables and undo the heater box from the firewall (four 10 mm nuts hold it on). (easy) From the drivers side, slide the bottom black plenium out. At this point there should be nothing from preventing the white heater box from coming out the right side with a little wiggling. Once you have the white heater box removed from the vehicle there are about 15 screws holding it together (you have to take them all out) easy. Replace the core (your new one may require a little foam or something to support it on the sides) I used door draft foam on a roll. The new one will not be an exact match to the old one. The heater box will be in three pieces - screw the whole affair back together, making sure everything lines up and operates nicely. OK - With a little wiggling you will be able to slid the heater box back in - (this is not easy) carefull with the copper pipes, a little easier if you have a buddy on the drivers side guiding it in place. (Hard) Replace the four nuts onto the firewall - this gets the box out of the way for the second most difficult part - putting the metal dash support back in (the one you trimed the 1/2 in from the top tab. Secure the two bottom 10 mm nuts and at least one at the top (small hands help) medium. Replace the front metal bracket (the one the radio goes into ( medium) - Connect the heater controls and snap in the cables. Replace the accordian ducting, plug the electrical connector (easy), replace the trim piece under the glove box (there will an air duct attached to it) (easy) put the glove box, ashtray and radio back in and then the radio cover (easier). Do the engine side hook ups, replace new coolant, connect the battery and hey you saved yourself about $500.00. The secret of whole job is the dash support brack must come out. Good luck, just had to share my morings activities while still fresh in my mind....Cam