Did this conversion on an 89 three years ago. I was able to use the threaded bung in a newer T-stat housing and a thermo switch to power the auxilary fan automatically, not just with the a/c on. The thermo switch provided a ground for the coil of a standard 5 pole bosch relay (only 4 of 5 poles used). The switch was closed at 215*F and would open at 195*F, it was an older Mopar application I believe for a K car and I'll research to see if I still have the part number.
On a parallel circuit a toggle switch mounted under the dash where I could reach it also provided a switched ground for the relay coil. When stuck in traffic at the toll booth I would just throw the toggle switch and the fan would come on. It is important to wire the + side of the relay coil to a keyed circuit so that you won't inadvertantly leave the toggle switch on and walk away from a parked vehicle which would kill the battery.
Also when doing this conversion I advise use of a newer style auxilary fan (96+ I believe) which has a different blade pitch & pattern. You can tell the difference as one has rather squarish blades and one has curled or swirl looking blades. Better air draw with the swirled bladed fan.
I miss the 89, sold it a couple of months after the conversion, :doh: but was happy I did this conversion to it.
Good Luck,
Tom
