Hi Maui
The fan is controlled by a temperature switch mounted in the radiator, so your best bet is to simply add a circuit that will allow you to bypass it. Use a couple of spade terminals and a terminal that will split the current from the temperature switch in the radiator. Using enough wire to run from the temperature switch for the fan in front of the radiator, you can allow the fan to operate normally but be bypassed into an On position from the cabin. The switch is on the right side of the radiator and has two spade lugs on it--carefully pulling off one of the female connectors and touching the other with it will make the fan run with the key in the on porition. Tap into the circuit (either side if fine) and run the line to your dash switch, leaving the other connections essentially as is. The fan uses quite a bit of current, so be sure to use a fairly heave gauge wire and adequate switch. As any speed over a trot the fan is a negative and actually impedes air flow, so beware of that.
Steve

If you mounted the fan in front of the radiator, it must blow towards the engine. A "pusher" installation.

On my 1988 240, the horns are activated (1) when the key is in Position I or II, and (2) when the steering wheel switch completes the ground side of the circuit.

That means that one of the wires to the horns is hot "all the time" (key in) and might serve your purposes. Also, that wire is fused.

I don't know the amp load the fan motor draws, so that idea might not work if wired directly. If not, a relay in the circuit could help.

Good Luck,

Bob

:>)