A thermostat housing from a later model 4.0 - like a 2000 or newer, has a temp sender hole that the earlier ones don't. It's on the top, so it's getting the hot water from the head, not the pump. The housings are the same mounting. Might be a good place for the fan thermo switch.
I just replaced a fan in a 90 Cherokee - only 2 wires - black is ground, white is hot. If they are reversed, the fan runs backwards. The fan cost me $5 at a wrecking yard. May be the simplest solution.
On house wiring and things like that, white signifies neutral - black is hot - I heard it started that way because black is the burnt color you'll be if you reverse them.
In Automotive, black usually signifies ground. Same for most electronics devices. Decent parts houses carry all the variations of switches you'd need for a fan installation, and they don't have to be mounted in J boxes. No, JC Penny does not carry them, neither does 7-11.
Most of the argument here was based on house/building wiring - unrelated to hooking up a fan. Fortunately after the fan is installed on the YJ (that's a Jeep Taz,) you don't have to call the building department to to get it inspected to make sure you had a permit and did it to code.