Part time trouble-shooter?
https://www.harborfreight.com/circuit-tester-set-3-piece-63612.html
and/or this:
https://www.harborfreight.com/electrical/electrician-s-tools/multimeters-testers/7-function-digital-multimeter-59434.html
At each non-functioning light 1) do you have voltage, and 2) do you have a good ground. No ground is easy to fix.
No voltage means you need to go upstream to make sure the 12V is coming from the switch, in which case you have a bad connector, or a broken wire.
Multiple 'faults' likely means the bad common ground, or a connector came undone.