The first thing I would do is pull out the climate control unit and check that the connectors are clean of corrosion and firmly plugged in. If possible try another unit. If that doesn't solve the problem you'll just need to work through the system as detailed in Toyota's service and repair manual (TSRM) for the climate control system:
http://www.cygnusx1.net/Supra/Librar....aspx?S=AC&P=1