Pontiac Grand Am

Hello, 1993 Pontiac Grand Am SE, 3.3 V6 180,000 miles. I have no heat at all, or very low heat output. This started when I flushed the system out of the petcock valve. I have changed the thermostat twice now, last time I installed the " open" failsafe type. I do not see any leaks whatsoever, and I have also bled the system for any bubbles that might be present. The same thing happened to my 93 Olds Achieva, same engine 130,000 miles. I am stumped on this one, any help is much appreciated.
November 18, 2006.

Thank you for your help, I believe the heater core is clogged, checked yesterday after running it, the hose is alot cooler, not hot in the least. Thank you again : D

Nov 25, 2006.
I have the same problem that just happened recently but my car is over heating but it's 10 degrees outside. In the last month my radiator light has gone on the fluid was low so I put more in the radiator, then there was a pooping noise when I brake and now it heppens when I accelerate. What could this be? Please help

Dec 7, 2006.