[cheshired5]

Assuming the cooler passenger side is a flap/motor not directing enough warm air, it sounds like a lazy coolant thermostat not opening soon enough and/or wide enough.

After you next go for a run, check both heater coolant pipes at the back of the engine bay and see if they're equally too hot to hold.
If they feel the same high heat, the heater core is circulating fine.
If one pipe is noticeably cooler, the heater core needs flushing and the thermostat will hopefully be ok.

Also feel the radiator top and bottom hoses.
The top hose should be too hot to hold all around it.
The bottom radiator hose should be cooler but cool.