'91 Sub heater core
 

Has anyone had the pleasure of changing a heater core in one of these beasts? Mine has air, just to add to the excitement.

I haven't done it myself, but you can find instructions with photos here: Colorado K5 Heater Core Replacement

Good luck!

Yes, I've done it. It wasn't too bad. Use the previous link for info. go to your local chevy parts dealer and ask for a line drawing of the whole assembly. It took me about 4 hours total and I'm no mechanic. I think it took longer to remove all the heater/defroster junk than the actual core. Good luck. walleye1 ps... remember to drain down your antifreeze or you'll get real wet and sticky!