Three wire-one is indicator/needs proper load to work.
I do a lot of electrical stuff, its hard for me to wrap my head around it too.
First go round with signals was wiring a universal switch into a M-37 for a buddy.
The front and rear signals are isolated from each other for brake light function. Just to make it even more confusing. A good test light is the best. Turn off the meter and put it back in the toolbox.
