How to troubleshoot uneven heating in an Audi 80?

My Audi 80 seems to have uneven heating, with the driver's side being significantly warmer than the passenger's side. What could be causing this issue and how can I fix it?

You may have a faulty heater control valve, which regulates the flow of hot coolant to the heater core. I recommend having it checked and replaced if necessary.

Check the blend door actuator, which controls the mix of hot and cold air in the heating system. It may be malfunctioning and causing uneven heating. Replacing it should solve the issue.

Have you recently changed your cabin air filter? A clogged or dirty filter can restrict air flow and cause uneven heating. Try replacing it and see if that helps.