I had the same thing in my '98 Safari. Using a factory schematic and a volt meter I chased the problem down to a bad relay under the dash. It was a real rats nest under there. There was a cluster of four relays and I found out which one was the right one by metering the connections while operating the dash fan switch. The relay was available at my local auto parts store.
Tom M. 1998 Safari Serengeti 3706,
300HP Cat 3126 Allison 3060, 600 watts of Solar.
Fisher and Paykel Residential refrigerator