hooking directly up with jumpers and battrie

If your using a fan that doesnt draw much power, you could probably get away with doing that. BUT..

It would probably be in your best interest to use a relay. The size of your fan depends on how big to use. A 30 would probably get the job done, but cant be certain unless the fan your using is known.

If you run a search on electric fans, you'll probably find much better info.

If you are sure its a good ground then yes you can. But I don't think you are sure if its a good ground. Try leaving the ground and jumping power to the red lead. Be careful of your fingers around the fan. If that don't work leave it connected and do the same for the neg. Once you have both connected right to the batt. and still no spin = bad fan or fuse/diode if it has one. It'll only take a second to see if it works. Good luck.

You should use a relay, or you will need 10 ga wire or something, and a 30 amp switch.

I just put a temporary setup in my 2.3 LX. I used a 30 amp relay, and connected both sides of the power to the battery, with a 30 amp fuse in the plus side. I connected the switch ground to the body, and used an old phone car adapter to get power from the cigarette lighter. Worked for 2 weeks.

If you permanently wire this, I suggest using a switched power wire for the switch that turns the relay on, or you will accidently leave it on and run the battery down.