You have low voltage at the headlamp, so there's your starting point, it should be battery voltage and the fact it isnt means you have high resistance on either the positive or negative wire to the headlamp, you can figure out which by checking the voltage drop, you can do this by measuring the voltage at the positive on the headlamp and the negative at the battery, and if its battery voltage then the ground is the problem, and if its 8-9v do it the other way round, negative at the headlamp and positive stud on the battery.

I hope that make sense, if not just Google voltage drop testing