As far as the sensor for the low speed relay goes we ran it into where the upper radiator hose connects to the radiator.
The reason I had the low and high speed controlled in a relay box is so that there is constant power to the box with the ignition on and you don't need a switch in the dash. You don't want low and high on at the same time so that you fry your fan motor. To me a dashed switched fan biggest problem is, not turning it off, but forgeting to turn it on...... Don't ask me how I know this, Make sure you run in-line fuses. Now that should bring you up to about $31.50....
LARRY