Help plz :(

I have a 03 monte carlo SS and it has manual dual zone climate control the passenger side will blow out hot or cold air ( which ever i choose ) but the driver side will blow out only hot air and no cold air... the other day i was driving and i had the drivers side set on cold but it was blowing hot then it randomly started blowing cold like 30 mins later...Then i purachased a new dash unit for my car and it seemed to work it went cold when i wanted it to and then hot when i wanted it to. But this morning i was driving to work and i switched it to hot it switched no problem. when i came out of work on the ride home i was getting hot so i tried to switch it to cold and the drivers side stayed hot while the passenger side went cold.. PLZ HELP IDK WHAT TO DO

A shop manual is recommended here to eliminate guess work, the temp door actuator on the drivers side may be the issue.