'96 buick regal- heater not producing warmth. fan blows fine and i did check passenger temp contol(set all the way warm) with no results. next best check???

Check the temp of the heater hoses going into the heater from the engine in the engine compartment.
If you have a temp gauge on the dash does it go at least 1/3 the way up the gauge from cold in normal driving.
You can take a garden hose and flush the heater....both ways back and forth then reconnect heater hoses and check for heat.
But.....The # 1 cause of no or low heat is the engine thermostat.
It needs to close tightly to force the engine coolant through the small heater core.

The thermostat needs to be replaced.........sodeep