I was thinking the same thing for my w201 but just for the aux fan, the main fan works fine left alone, but when you got no a/c you can use the aux fan to do this. The easiest way would be to simply cut the wire from the relay and solder one from a switch that is in the cabin running through a fuse in the electrical center, just to be on the safe side. Then you can flip it on and off whenever you like and it will shut off just like all other parts when the ignition is off.
But I'd prefer not to touch the original circuit and implement something into it, however, since I'm not an electrician, I'm not sure how to do that yet, but one day I might sit down and figure it out just for the heck of it so I can switch my fan on and off cause by itself it never goes on.
1985 190E 2.3L - a constant project.