Its a complicated system. Good luck. I'd start with the hot side. First check the heater coolant valve for the heater core in the engine bay. Its known to fail.
You have a mechanical slider in the passenger foot well area that tells the temp blend door servo what position it is in. Take that off, clean it with some electrical cleaner, use some high quality dielectric grease on that slider, pop in back in and you should be back on your way. By this time all the factory grease is most likely all gone and the electrical contacts in these servos should be cleaned/serviced to help extend the life. I had to do all of this on my GS300 as this is a common problem on this vehicle.
EDIT: Here is a auction with a picture of the mechanical slider in the middle for reference (with the brown rectangular piece with a circle on it).
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