I have a whirlpool package unit aprox 20yrs old. Can't find a model#. The blower runs OK when I put the fan setting to on, but won't run when I set it to auto. The compressor kicks on and off when it is supposed to as I change the thermostat setting, but the fan won't come on. A couple of times the blower fan came on for about 30 seconds then went off, but started right up when I flipped the switch to on instead of auto. Now the blower will only come on with the switch set to on.

Most likely your thermostat or subbase is going bad. If you have a volt meter check the voltage between the "C" and "G" terminal in your air handler with the fan switch in auto position and calling for cooling. If it works in on and not auto there's not much else it can be because the thermostat energizes the G terminal bringing on your blower.