A couple summers ago I spliced the ECM fan wire (green wire from the relay) and ran a wire to a toggle switch in the dash. Works fine.

Now, my temp switch controlled fan isnt working. Thought it was the motor. Replaced that and still not working. Now I believe the temp switch in the block burned out.

Rather than replace the switch (kind of a pita) can I just run a wire from the connection at the temp switch to the same toggle in the car and be able to manually turn both fans on at the same time?

doing the temp switch from under the car is the fastest way and easy to. Well if your car is a V-8 at least ... provide a ground with your toggle switch to the fan relays green wires and they will come right on.

It is a v8. I already have a ground from my toggle to the secondary fan relay up by the firewall.

Sorry to make you clarify, but does the primary fan have a wire at this relay also?

I spliced into the wire that comes out of 20 amp fan fuse, ran it to a switch and then ran that to the hot side of both fan and grounded the fans to the frame. That worked fine until I had to upgrade to a higher output fan and shroud setup

thats how mine is done,but both fans draw alot,headlights dim a little with both on,so i just unpluged 1 of them,dont really need both on,i have no a/c

Make sure you are running your switch on the CONTROL side of a relay, not a side where you are pulling high current, as the switch will get hot and melt. Other than that, yes, you can run almost anything on your car with a switch.