Why do I get heat on left side and not on right

Check your actuators on heater, they caneither be electric or vacuum. One of htem may not be working. If electrick try puling hvac fuse for 60 seconds sometimes they reset

There is a problem with one of the air vent doors. The actuator for it has gone bad or the door has come off its hinges. The heater box will need to be removed which runs from the underneath of the dash from the center to the passenger side.