We have been testing several brands and types of fan controllers to try to replace those on the 1987-95 cars that are failing.
It seems that either the fan amplifier unit (under the hood, next to the hood latch) or the fan controller (mounted in the car next to the passenger seat on LHD cars) are burning out with no reasonable replacement available. I know that its getting harder to find a good used one, too.
We found a unit that can control any number of fans adding up to 65 AMPS, it fits where the original fan amplifier unit was mounted under the hood, and it uses the factory fan wiring. I've installed it, tested it, and written up a good set of instructions for the 928'er to install it with. This model uses PWM (Pulse Width Modulation) technology to start the fans at 60% and then increase fan speed as needed as temp rises; decreasing fan speed if the temp falls (like on the interstate) - providing excellent temperature control.
I believe I have the kit priced right - I am coming to market at just $198.15 for this, which is well below any OEM replacement. Again, it replaces BOTH the Fan Output Amplifier PN 928 618 421 00 and the Fan Controller PN 928 618 418 00 with one easy-to install and wire controller.