Why not just fix it?  Find the temperature sensing switch.  Find out
if the problem is the switch or the wiring.  Replace the switch if that
is the issue, or fix the wiring if it isn't.
--scott

It doesn't make any sense to use a toggle switch to control the fan. The
switch, power relay, wiring, and time to connect it all will surely cost
more than replacing or repairing the component that has failed. Besides
that, you are bound to eventually forget to turn it on and fry the engine or
forget to turn it off and kill the battery. There is a reason it is set up
the way it is and if you defeat that system you will surly cause problems
eventually.

Having said all that, if you must use a toggle switch be sure to also use a
power relay because the fan draws too many amps for a toggle switch.
You buy a relay (typical fog lamp relay will do). Attach it under the hood
near the battery. Run a 20 amp fuse link to the relay power terminal. Run a
12 ga.  wire from the relay out put to the fan power terminal. Run a 16 ga.
wire from a spare fuse slot in the fuse box (that is only on when the key is
in the run position) to the relay input terminal. Then run a wire from the
relay control terminal to ground. That way you don't need a switch and the
fan will always be on when the key is in the run position.

Like Kevin stated, if you really want to toggle it.  However, Kevin, for
point of discourse, it's an '83, so many things up the channel from the
fan itself could be bad, and not necessarrilly the sensor that triggers
the relay.  I read OP's post as a fast fix to manually run the fan sans
the tracing and replacing of who knows what.

Had a '86 Celebrity with same problem, said too much b.s. to do it
correct, meaning too much $$$, so I just direct-wired the silly thing.

Caveat> I did have a temp. guage, so I could tell when the engine got
too hot.  But I used a switch with an inline 20-amp fuse, lighted when
on, so I knew if it was on when ignition was off.

As said, just for discourse, your thoughts?

There is a reason the fan won't come on - find it and fix it.  There
are many reasons to not do what you are suggesting.

If I wired in such a toggle switch, I'd probably forget to turn it on
and would burn up the engine.  :(   For me, it would be a lot safer to
wire the fan so it's always running when the ignition is on.

www.autozone.com has free repair information for some vehicles.

I would look for devices attached to the cooling system plumbing with
wires going to them.  One should be the thermostatic switch that makes
the fan relay turn on and off, and on my 1986 Corolla it screwed into
the aluminum cover for the cooling system thermostat (radiator hose
attached to it).  If you unplug its cable and short the cable's two
pins together (or, if there's only one pin, probably ground it), the
fan should run when the ignition is turned on, indicating that
everything is OK except for the thermostatic switch.  But if the fan
doesn't run, check the fan fuse, wiring, and relay.