No heat

You say new thermastat. Where was the new t-stat made? If it was mexico then it could be the new t-stat.I ran into this when I put intake/head gaskets in mine in '05. I put a new t-stat in cranked it up checking for leaks and noleaks but the temp hand went up 3/4 way and stayed there no heat. Rechecked everything after it cooled off radiator full then pulled the t-stat out went and got another one. Put it in same thing. Did this 5 times each time same thing it would warm up then when the t-stat was supposed to open never did temp kept going up. Just for the hell of it we boiled some water ste it off burner dropped the new t-stat in it never opened. Got to looking at it made in mexico, as were the other 4.I had to go to 8 parts stores to get one made in usa put it in temp gauge went half way t-stat opened run like a new one. Lesson learned t-stats made in mexico ain't worth the box they come in.

You probably have air pockets in the core. Have the cooling system pressure tested and see if it holds pressure for a long period of time.

You also could have gotten it too hot the first time and have a blown head gasket now. Very common on that engine.

First check your coolant level if low fill it up then have it pressure checked for leaks. It could be a head /intake gasket. Then pull the fuse for the hvac/heater and leave it out for 60 seconds with the key on. Then reinstall it. If still no heat have your air temp actuator checked to see if it's working or not. It may not be.