Where do I start?

Nope. The laws of physics have been rewritten and switches and relays no longer work because they were too reliable. There is a power control unit in the dash. It's a computer module that pulses the fan motor with a varying "duty cycle" to vary fan speed. Duty cycle means the percentage of time the motor is turned on compared to off. That was done in response to people not being happy with just four fan speeds. The computer can vary the speed from off to full speed and anywhere in between. This is critically important, and this type of insane reasoning is why I refuse to give up my '88 Grand Caravan and buy something newer.

As with any computer module, they have a high failure rate. Heat is also the deadly enemy of anything electronic, so you know it's going to have problems. That module sits near the bottom center of the dash. Check for loose plug connections, but suspect the problem is inside the module.

Very happy to read it's fixed. I helped a friend rebuild a smashed '07 Dodge dually diesel last year and the integrated power module cost him $800.00. Only the backup lights, motorized pedals, and 4wd lights didn't work. I easily bypassed the lights and pedal problem by wiring in simple, reliable, inexpensive relays, but he had to spend the money to get the 4wd lights working so it would pass the state inspection. Thank you, you insane engineers. It's no wonder there are so many people who don't want to own new cars.