Can't offer any advice on an 89, but on a 2004 Dakota....I had to drop the whole dash to get the heater/AC box out. I was doing the job the opposite way, with a bad heater core. I dang sure wasn't taking a chance on having to do it again, so I replaced my evap core while I was in there. If it has electrical door actuators, note what the dash control setting is when you take the box out, and take pictures of each actuator position before disassembly. That will save a lot of head scratching when you put it back together! Best of luck to you.