My 2000 Monte Carlo is blowing cold air out when heater is on.will warm slightly when car is moving but goes back to cold @ an idle...?

SOURCE: engine overheats when heater pipes connected

I think you could be right about there being air trapped in the system.
Or the heater core(radiator) is plugged.
Either way you need a pressurized radiator coolant pump to put the coolant in without air being trapped.
Although,i have seen it done by jacking the vehicle up high enough that the radiator was now higher than the heater core level, and then the coolant was put in.
But this is dangerous to try.

SOURCE: 94' Maxima - I warm up my car and the engine shows

Hello,it seems your car wont stay warm while driving, this is going to be a rather easy and inexspensive fix the thermostat is going to be stuck open. You need to replace the thermostat. Thank You

SOURCE: 2004 Tahoe Heater Problem

check coolant level ,must be low ,chck for coolant loss

SOURCE: heater blowing cold air

My 2005 Nissan Sentra 1.8s with 55,000 mi. blows cold air from the heater. Coolant not circulating in engine. Lower hose stays cool to touch. Both heater hoses cold. Changed thermostat - didn't fix it. Coolant not leaking or dirty. None in oil - no oil in radiator. Changed worn water pump belt - pump spins freely with no noise or resistance. Cooling fans come on after warm-up. Temperature gauge stays in normal range then jumps to hot and back again. Coolant good to 10 below - didn't freeze here yet. No other problems or repairs to car. Afraid to drive and make worse. Help!

SOURCE: 2000 Xterra intermentantly overheats

sounds more like the thermostat may be failing. generally a water pump starts leaking out of the "weep" hole in the casting before it quits. just look at the front of the water pump for a greenish streak or an obvious leak.
if you also suspect you heater core may be getting plugged now would be a good time to back flush it to get rid of some of the gunk.