simple load light out? Headache!

Is the the top of cab fiberglass? Try running a wire from ground to the mount.
With the muti-meter set it to continuity or the lowest ohm setting,touch probes together see what it reads. That's what you want to see between fuse and switch.

 

Run a longer wire to the original ground location.

 

You should have power to the switch. How are you testing for power?
How is the light grounded?

 

If you don't have power to the switch, then it has to be a problem between a power source and the switch. Because it was working before you made the change, I'm thinking there is a fuse somewhere that you have missed. You probably blew it when you were changing the lights.
You need to start tracing the hot lead from the switch back to the power source. If that doesn't work, then you need to trace the ground lead back to a ground.

 

in line fuse. Fusible link.

 

From what I know aluminum is a bad ground.(Google) I would look for solid metal though first you can test this by just extending some wire to the frame and see if anything happens if not then it's not ground.

 

If there is no power to the switch, he can ground all day and not see the light...

Use a non destructive tone generator and check the feed to the source...or go see someone that understands electrical circuits...

... Philip

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