Sorry about giving this thread a Necro-Rez, but I've been having the dreaded "Intermittent Heater-Blower-Fan Syndrome" for about a year, on my 1991 XJ Laredo(4.0L, AW4 auto-trans, NP242 xfer-case, D30-front, D35-rear, etc., etc., etc.), and found, in my case, it was just the connectors to the blower fan motor & resistor pack that were either dirty, mildly corroded, or 6-of-1, 1/2-dozen of the other. A few shots of Amsoil MP, from a can I've had sitting around a few years, and unplugging, and replugging in, each connector a few times, seemed to do the trick of both cleaning off the dirt and/or corrosion from the contacts, 'cause now the fan comes right on when I set the Function Selector to Heater, Vent, or Defrost(no A/C though, as the A/C system's components have been completely removed from my "HEEP"), as it used to take a few seconds for it to come on before, whenever it chose to work. If the problem comes back, I'll likely re-check those connectors, as well as the multi-function switch & the blower fan speed control rocker switch connectors as well. If I've gotta replace anything, I'll start with the resistor pack, as they do get really nasty after a few years of doing their job. After that, time, and further testing, will tell. Hope this helps, maybe...