heat works fine on passenger side but not on driver side but the blower works on both sides why

what model year? dual zone climate?

If the vehicle is 95-99 Riviera the problem most likely is the item(s) called actuator Right. What this item does is open when one engages the the heater switch. It appears the Lt. actuator (door) is working. In order to view the location of the part..please visit gmpartsdirect.com using the search engine locate your vehicle..locate HVAC / AIR CONDITIONER & HEATER / EVAPORATOR & HEATER COMPONENTS . The part in question is No. 18. Good luck because it takes time, know how , certain small tools and patients tp replace those parts.SEMPER FI