Chevy Trailblazer headlights don't work + HDM input/output testing

If BOTH headlights go out at the same time, this is a good test procedure.
If swapping the relays (aka HDMs) makes the headlights work, replace the HDM part number 15016745.
Note: I am using a computer safe test light in this video. It is not recommended to use a standard incandescent test light for these tests.