I would personally check the heater core, it could be plugged. (and I know it's a new thermo, but even brand new, never used, never messed with thermos don't work as well) I would also check the themo housing, If sealer got in to the thermo it won't work right, and you will not get much heat, if at all.
Thermostat housing diagram for a 2000 Pontiac grand am?
do you have a diagram for pontiac grand am 2005 thermostat
The thermostat is located near the intake on a 1999 Pontiac Grand Am GT. The thermostat can be located by following the upper radiator hose directly to the thermostat.
Check coolant level Could be thermostat is not opening Could be a bad water pump
How do you change a thermostat on a 2001 Pontiac Grand Prix 3.8 gtp
Where is thermostat in 2000 Pontiac lamans
how do i change it
the thermostat is right under your throttle body and it is difficult to get to
with tools
To replace the thermostat on a 1997 Pontiac Grand Am 3.1, first disconnect the negative battery terminal and loosen the clamp on the radiator hose. Next, remove the mounting bolts from the thermostat housing and insert the new thermostat.
in the water pump
If a 2000 Pontiac Grand Am shuts off while in drive, there could be a problem with the passlock sensor. There could also be an issue with the fuel pump.