a/c problem on 67

[easyrider]

I am having problems with my a/c. The clutch isn't engaging when I turn on a/c fan. I know on the 67 you have to put the lever on a/c and on cold and then turn on the fan to make it come on. The fan comes on. With all that done, clutch still doesn't engage. I made sure the fuse was good and that the plug on the compressor was plugged in good. I have a relay in the circuit and there is the fan switch itself. Any suggestions as to where to start the tracing process? What is the most likely thing to go wrong. I had the a/c serviced last summer and the truck has set in the shop all winter. Yesterday was the first time that I tried to use the a/c and found that it didn't work. Any suggestions would be appreciated. Thanks!