Radio shack has a nice standard 12V/40A relay for $6/7...

What you do is take the single wire off the R15 dropping resistor behind the headlamp and use that at a relay to trigger the relay [ term 85/coil]
Ground the other coil terminal to complete the coil circuit [ 86]

Now , use the power contacts of the relay [ 30 and 87] as a 12v power feed from the battery to 30 and 87 goes to the R15 2 wire side . Put a fuse in the bat feed line to relay [ 30A ] ...use heavy wire for this line..
That's it. Now , when the high side pressure sw at the reciever/drier closes and calls for low fan , that sig triggers your new relay instead of the dropping resistor, and the relay BY-PASSES that R15 ...so you wind up with high fan for ac instead of factory low speed...that's all.
All other circuits stay the same ..

Some guys just jumper the R15, but that fails real quick b/c the factory wiring for low fan circuit is too small to be running a HIGH fan/s current draw... see them burnright up ..[ but the addition of the simple relay/circuit will take that extra load/draw needed to run the fan/s on HIGH.]
You can also jumper R15... IF you double up the wire from the low fan relay to the single side of R15, and then put a high fan relay in place of the low fan relay. That is more work and cost, but also works.
Either way does away with the dropping resistor R15, which is what gives the fans their low speed.
If you are in moderate climates , low fan is adequate..

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A Dalton