Car A/C problem. Light comes on but Compressor doesnt engage?

I have a 1991 Toyota Tercel with factory a/c. When i push the A/C button, the light comes on but the compressor doesn't engage. Now it uses a magnetic clutch to engage the a/c. The first thing the Haynes book says to check is the relay switch way the hell up under the dash. But im wondering, if the light comes on, is the relay switch still good? If the relay switch is good, then what should i check next? Thanks for any advice/ help. -Robert