That what I did on my '99 S10 (V6 4.3 L) two days ago.I pulled two heater hoses from heater core,you can see them sticking out from firewall.Put lot of rags on the top of distributor cap(not to get it wet) and run shop air trough heater core.O boy!Lot of s***t come out of there!Kept running air for a minute,then put hoses back,refiled with coolant and (OH GOD!) I've got HOT-HOT air again from my vents.YAHOOOOOOOOOO!