[kyleb]

The wire you are looking for is small gauge, just enough to ground the indicator light on the dash

On mine, the screws were brass. I was replacing it on a gravel floor. If you drop one, a magnet won't be any help.

Since my switch was on the 'wrong' side of the air tank, access was very difficult, I could see it, or I could touch it - but not both at the same time.
It took me over an hour to find the darn screw!

If I ever have to replace that switch again, I will relocate it for easier service!