Originally Posted by katkandu
I looked around, no ground wire I can see. I suspect itís not there because these lights are also able to be shut off with a breaker switch on my 12v panel.
Ahh.... no, your suspicions are incorrect!
There might not be negative (ground) wire near the switch but there will be one at the LED light fixture itself.
But it doesn't really matter much where you find a negative, any one of them will will work.
To recap rudyk's advice - the dimer needs a positive source and a negative return (the red and black wires) before it will operate. The output of the dimmer (white) needs to go directly to the LED positive side. The other side of the LED will be the negative return going back to the negative (ground bus).