provide ground when powered

I have an older car with a newer engine, and I want to have my fan on my engine turn on when I turn on the A/C. I was thinking I could run a wire to a relay from my compressor power wire that would provide 12v when the compressor comes on. I would like that to trigger a ground at a relay, which I figured I would wire into the ground on my fan relay. Sound possible?

I've looked at the diagrams on relays here, but don't see a situation for what I want, and don't know enough about wiring to figure that out. Hoping for some direction.

So, hot wire from A/C (when A/c turned on) to what pole on the relay?

Ground wire from my fan relay to what pole on the relay?

I am thinking power to the relay to 30.

Ground to the relay to 85?

What type of relay? SPDP?

Many thanks!

For switching the negitive look at this.

Thanks, The 12Volt Administrator. This worked perfect, and providing me with specific wires to specific terminals was very very helpful.