It's possible that the thermostat was stuck open and wouldn't allow the coolant to heat enough to let your heater operate. It would be kind of like not having one at all...the heater won't work. Depending on how old the heater core is, I'd be hesitant about forcing too much pressure through it, in other words, I'd be hesitant to "blast" it. You risk blowing out any weaknesses in the core. You didn't say what your coolant looked like. Maybe just replacing the thermostat and doing a reverse flush on the cooling system with the heater on will do the trick. You can still get the kits for about $5 at discount stores to do a flush with a garden hose. It works pretty well. Also, make sure that your heat select switch is opening the valve that regulates how much hot coolant goes through the core. The good thing about the ones on these cars is that it is mechanical and easy to check and fix, if necessary. A good rule to follow, as I found working in garages as well as at home, start simple and then work up to more complicated. I've thrown many a tool across the floor because I didn't follow this.