lets start with the bike not running, with the ignition off take off the connector from the headlamp and using a multimeter on continuity find out which connector is ground and verify that it is a low resistance to the ground or metal parts of your frame etc, or your battery neg connecion. were looking for almost a dead short, make sure that it stays the same in both high/low beam switch positions.
Next turn on ignition and switch your meter to measure DC12v and with switch in high beam, measure from the socket terminal you know to be ground to each of the other socket terminals, you will get 12v at one of them as you know the high beam is working.
Switch to low beam and repeat to verify if youre getting 12v at the one remaining socket term to your ground, according to your symptoms you wont be getting this, if this is so we'll take it from there.
Tip..If you cant get your meter probes into the socket to make a good contact, then unwrap a paperclip that is small enough to shove into the socket terminal and measure to that.
Hope i'm not telling you how to suck eggs with this!
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