If you have a meter you can measure resistance at the headlight terminals. The highest resistance between any 2 terminals will be between the hi beam and low beam terminals. The third terminal will be the common (ground terminal) Hook your ground from the bike harness to the common terminal. Then each of the other 2 will turn on the either high or low beam. The low beam should have a higher resistance reading than the high beam when meaasured to the common terminal.
I suspect you may be connecting between the high and low beam hot terminals and then switching to one or the other to ground. Find the common terminal first and go from there.
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville