tail light/electrical problem

[mbfiretech]

I've been troubleshooting the electrical to get my 63 street legal. I get headlights when I pull the light switch out 2 clicks. When I pull it to 1 click, I can meter 12vdc on the wire at the rear, but when I connect the tails, the voltage drops to 0vdc. If I run a 2 conductor wire straight from the battery, along the floor and then to the tails, they light up. What would drop the voltage? Thanks for the help. Also, what is the operation of the switch? What happens with one click, and two? Thanks
Mark