Fan control and AC problems

I have a small problem with my fan control and AC on a 1994 Nissan. It works intermitently. Somtimes it runs and others times it does not. There is no obvious combination of events to provoke the problem. As long as the fan runs the AC and Heat will run. If the fan does not run, then nothing runs, heat or AC. Any ideas for solutions are very much appreciated.

About once a year my (93 pickup) fan relay starts acting up .. no fan or ac. There may be a "root" cause for the relay failures, but so far I've gotten away with relay replacements ... lasts for one summer or better.