What's the best way to wire the auxiliary fan to a switch. '99 Cherokee sport. Can I just run it off the battery?

 

Question 1 is why? You could just splice a switch into the relay at the pdc if you wanted it on a switch that badly.

The fan comes on automatically at 217 degrees no matter what. That's only 5 degrees above the normal operating temp.

 

Basically the why is that I don't think the fan comes on. The jeep has been running a little bit hotter than it used to and I've had a few times where steam has come out of the radiator cap. I changed the water pump and thermostat and radiator cap but I'm just worried about it happening again. The temp doesn't seem to get over 210 when I see the steam. I flushed out the radiator and it seems fine. It looks brand new.

 

Why don't you try testing and repairing instead of thinking up a halfassed solution?
Start at the fan and work back. Hotwire fan first, if it works move to checking the relay, etc.

 

Verify the fan is working first. Turn your ac on and the fan should cycle with the ac compressor as well. If its not working the problem is either the fan or the relay. Swap relays to check the relay and take the fan out and hook it to 12 volts directly to see if its working.