To find it, you need to consult your owner's manual. This manual will have a diagram of the fuse and relay boxes, showing the location and function of each relay. The relay will likely be labeled, or you can trace the wiring diagram for the AC system to find its controlling relay.
If you don't have the owner's manual, you can try searching online for a 1991 Toyota SR5 wiring diagram. Many websites specialize in automotive repair information, and some offer free diagrams (though often with a subscription). Be sure to specify the exact engine size (e.g., 2.4L, 3.0L) for the most accurate diagram.
Without the manual or a wiring diagram, physically searching is difficult and time-consuming. The under-hood fuse box is a good starting point, as most relays for major components are located there. Carefully examine each relay, comparing its size and shape to what you might expect for an AC relay (it's usually a larger one). Be cautious when working around electrical components; disconnect the battery's negative terminal if you are unsure.
