Suddenly AC and heat are inoperative. It's on a friend's 91 4 Runner (I
think). I checked as many fuses as I could find under hood and behind
the glove box. Is there a relay that controls both and how do I check
it? Any other suggestions?

 

there are only the two relays(if you have rear heat) that actually control the fan the is also 2 fuse's and an overside fuse(it has a green top) in that same link. they are under the glove box. I had the same problem and it turned out to be my high low switch itself was bad. every time i would turn it on all i would get is a click sound from the relay breaking. check the oversized fuse first if there is no clicking sound.