EnZI

I have a 2004 RX 330 with 66000 miles. I have problems with the A/C. The Driver's vents are blowing hot air while the Front and Rear passenger vents seem to be blowing cool air only when the temperature outside is about 72 degrees. I took it in for the repairs and the mechanic told me he suspects I have a blend door issue which would cost about $1000 to fix. He did not perform the diagnostics (which cost $180)Before I invest such an amount can anyone offer susgestions?