Well, there's a few benefits to having the fan running on low low all the time.
Mainly, it gives a little positive pressure in the cab. This makes it less likely for things like dust to make it in around the lower door seals and helps keep exhaust fumes out. It also pulls a little heat out of the heater core as there is no bypass valve for the core.
My 98 is off when it's in "off", but I've always go it running on "low" pumping air over the windshield anyways.
Options to disable?
You could install a small switch in the fan ground line and flip it off when not using it. Keep it within reach of the driver as it would suck to have to stop and reach over to turn it one when you need it.
Another option (since the heat is your main concern) would be to install a heater core shut off/bypass valve. That way you can turn the heat off to the core but leave the fan running. Many OTR truck models have a heater core bypass incorporated for this very reason.
Now that I think of it, it's entirely likely that a bypass might even improve the AC effectiveness. Never looked at it that way before but that could be why OTR trucks have the bypass. I always knew it was there to help keep the cab cooler, never considered it might be to keep the air temp at ambient (instead of routing it through a hot heater core first) when it reaches the evap core.
Another use of a bypass valve is the ability to shut the heater core off should it develop a leak. You'll still be swapping the core out, but shutting a leaking one off affords you the ability to get it home or maybe still use it to get back and forth to work until the weekend arrives and you have time to swap it out. Heck, if it's summer it can give you a couple months to save up the money if you don't have it right away....
These are all interesting points and now it's got me thinking of a new way to incorporate my cooling system bypass filter assembly.....
I know the older square bodies with AC had the same always running scheme. One of their solutions was to bend over one of the resistor assembly tabs and that disabled the "always on" feature. However, I have no idea if that would work on an 88-94 truck. Use with caution if you decide to investigate this possibility.....you just might "let the smoke out"....
