I have to change the wiring from my old incandescent bulb to the 3 prong H4 connector. How do I know which is the high/low/ground wire?
When I've changed sealed beans for halogens, the connector on the back of the sealed beam was the same as the replacement H4- that was for 5 3/4" round lamps though, other types may vary.
Hold the H4 connector in your hand pointing away from you (as if the bulb was plugged in on the other side, with the wires towards you). The vertical pin on the left is ground, the horizontal one across the top is low, and the right vertical is high.
Then find the ground wire - with the lights switched off and both existing lamps unplugged, set a multimeter to continuity mode, and check each existing wire to the negative on the battery. One will show continuity - I'm fairly confident it'll be the black one.
Then set the meter to 0-20v DC, and switch on low beam. Test between the ground wire and each of the other two - one will show 12v, the other zero. Repeat with high beam on and check that it's the other way round.