relays for electric fan in truck

I am planning on installing an electric fan in my 99 Silverado.  It will some on, triggered by the engine computer, when the engine gets to X temperature.   It will also need to be on when the AC is on irregardless of the engine temp.  The trigger wire coming from the engine computer provides a ground.  The trigger wire for the AC will be a tap into the positive line of the compressor motor.

I can figure out how to wire the relay with the trigger wire from the computer.  Since it provides a ground I could just pick up an ignition switched wire from some where.  These would be the 2 wires for the coil right? 

I'm stumped on how to work the AC wire into this since it is positive.

I can think of a simple way, assuming the feed of the compressor is adequate to run the fan, wire a single pole double throw relay like this

Pin 30 - The positive lead of fan (ground the negative lead of the fan)

Pin 87a- To the feed of the a/c compressor

pin 87- To a twelve fused source according to the fan's needs!

Pin 86-  True Ignition

Pin 85- To the ground signal from the computer!

Mr. Iidot or Mr Pierson or one of the relay guys will correct me if it isn't that simple!

Thanks.

I'd rather not run the AC compressor and the fan motor off the same feed wire like that.  If the feed wire could handle the current that would be the most simple though like you said

Here is what I came up with:

I'd suggest not running the main fan power through both relays like that.  Just use a small relay to invert the control signal and use the output from that for the main power relay.

Having the fan current run through both relays that way is more failure prone since you have two relays in series, and the 87A contacts generally have lower current ratings than the 87 NO relay terminal.

Here is relay diagram #2: