Hands on testing is needed. Something is going on with the mode door that directs air flow, or related wiring.
Use this link to find some shops near you that can help with your concern:
www.repairpal.com/repair-shops
RepairPal Certified shops can diagnose and repair your 2000 Lexus RX300. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.