I am looking into the same thing myself. I went down to West Virginia last week for a camping trip and started losing coolant onto the floor board. Not fun in the middle of nowhere. I took it to a garage and they said that for Isuzus, there isn't a core per-se, but a "hearter unit" that is $350 rather than the standard $75 or so for just a core. I have a 96 rodeo, does any of this make sense, or was he just trying to rip me off. Well, he couldn't get the "heater unit" until today, this happening last tuesday, so I told him I would think about it and bypassed the heater all together. That works for now, but I want to fix it before winter-obviously.
Sorry for the legnth, any infor is appreciated.