According to the wiring diagram on S.S., circuit 17 is the bright headlights & circuit 18 is dim headlights. It appears the they are the same wire coming fromt he light switch until they have to split just forwaard fo the dimmer switch. If the right headlight does not work then it must be physically on the right side of that splice of 17left to 17 right & 18 left to 18 right.
Circuit 16 coming out of the light switch is what feed the dimmer switch which routs power to 17 or 18 (bright or dim).
Circuit 491 is the single parking light power wire going up front that is also split between left and right.
Circuit 21 is park light for rear which is split at the left rear frame area in order to be routed over to the right rear.
I would think that you have a ground issue rather than a power issue.
Perhaps you can probe the circuits starting at the lights using a ground wire connected directly to the 2nd battery or the frame where the battery grounds are conected. If you have 24 volts at each light, then it is a bad bulb or a bad ground. The left side would not work at all if the light switch was bad according to my study but I hope others will chime in here too. The reason that I beleive this to be correct is that there is only one 16 wire, one 461 wire, and one 21 wire coming out of the switch so it would be all or nothing is the switch was the problem. New switches are about 30 to 50 dollars from several of the dealers or on e-bay.
The wiring diagram is attached to this as a PDF but is light green. Open it up and then zoom in as required (Hint save them to your desktop and then opern them).
It took me a while, but it is actually pretty easy to read after some studing over coffee or tea.
Good luck and please post your findings so that we may all learn more about our "mighty beasts of green".
