I'm battling the same issue... adding a manual switch brought me piece of mind.
Wiring in a toggle without altering the factory harness is easy enough. Get the crimp on tap that lets you attach a new wire to an existing one without cutting it. They crimp on with a pair of pliers and have a snap over cover to insulate the blade you just crimped over the two wires.
That tap you put onto the hot right at the fan connector. I attached to the yellow/low speed wire. The new wire goes to the car interior to your new toggle.
With a fuse holder, attach a second wire to the positive post on the battery and run that to the interior as well. I used a 20 amp fuse. Putting a fuse in there is just basic safety. Electrical fires are ugly, especially if it consumes your car.
The switch I mounted to the plastic shroud beneath the steering column. It is right between my knees, a tad awkward cuz I can't see it w/o leaning over, but there isn't any chassis metal in the way right there.
I bought a lighted switch so I had to add a third wire that goes to a nearby chassis screw under the dash.
When the switch is on, it lights up as a visible indicator.
The toughest part of all this was drilling through the firewall from the engine compartment. I actually drilled from inside to the exterior, left of where a clutch pedal would be. That whole area is crowded and I was concerned about drilling into something I didn't want to.
The whole thing only cost about $12 and I have extra wire and terminals for future use.
FOOTNOTE: I did the relays today and learned the pilot light feature of my toggle blows the 40 amp fan fuse! Doesn't make sense to me that it would do that... in fact I was hoping relay power feeding back on my new line would light up the switch, showing me when the fan was on via the factory system. It cost me ($3 x three) going over it all before I was sure that was the issue. Needless to say, I'll be monitoring it all for the next few days to be sure all is well.
Regardless, the switch will be staying for my own peace of mind. I do expect I could put in a diode on my feed to the fan motor and that would allow me to leave the pilot light feature working but right now I'll do without.