Heater will not blow hot air. It will only blow cold air even when the temperature is turned to high heat
You may need a new thermostat and/or heater core.
I presume the repair has been made. I helped a co-worker with her 99, same symptoms. This has an elaborate climate system, accessed behind the glove box. Allow several hours, and at least $160 for the new air mix servo, the likely culprit. There are 2 others, the mode servo (floor/vent/defrost/mix), and recirc servo, less likely, since heat is the issue here. here is a forum link, a little googling will show the process. http://us.lexusownersclub.com/forums/topic/71031-no-cabin-heat-2000-rx300/ The Lexus dealer charges $500 and up to fix this.
Looking for a Used RX 300 in your area?
CarGurus has 849 nationwide RX 300 listings starting at $2,500.
Search Lexus RX 300 Questions
Lexus RX 300 Experts