[dsraven]

if you have power out of the dimmer on both circuits, separately I assume like not at the same time so low beam is powered when called for but high beam is not powered when the low beam is called for. if both are powered up at the hi/lo switch at the same time then go and unplug the headlights and retest the circuits coming out of the hi/lo switch. if it is normal now with the headlights unplugged, only one of the circuits powered at a time, then the problem seems to be a bad ground at the headlights which is back feeding through the headlight elements in an effort to find a ground. with that sorted out go to the actual headlights, unplug them both, so no chance of a bad circuit in one side contaminating the test results from the other side, and see if you have power there for each circuit. turn the lights on, without the headlights connected at the plug ins, then take your test light and ensure your tester is working by grounding the test light to something that is an actual good ground, don't rely on the ground at the headlight plug, then test the test light's operation by testing something that you know has battery voltage, like the battery if your tester cord is long enough. if that is good you know the tester works now. try each connection on your headlight plug. one should be hot since your headlights are turned on. make a mark on the plug or a quick paper diagram of which connection that is. now go switch the hi-low switch to the other position and go back to the headlight plug to test for power. a different terminal should light up the tester and the previous one should show no power. since there are 3 connectors on the plug it would make sense that the last terminal is the ground. a simple test for that is a reverse check where the tester is powered and you look to see the tester light up when you touch the ground terminal of the plug. if good try a different load on the circuit instead of the tester or a headlight. use a set of jumper wires so you provide the ground separate from the ground in the headlight plug. this is because sometimes a circuit will light up a circuit tester but a bad connection in the circuit won't allow enough amperage to light up the actual headlight light. or, your high beam is powered to the headlight but can't find a ground so the power is going through the elements inside the headlight, which share the ground, and back out the low beam circuit where it finds a ground somewhere in the lighting system. sometimes when the lights are switched on and you have a bad ground scenario you will see a turn indicator light up on the dash even though the signals are off. that is because the system can't find a ground at a marker light so the power goes through the bulb elements and back out through another circuit, like the signals, and then finds a small ground in the dash light bulb. that bulb can't supply enough ground for the system so nothing else lights up except for the dash light. you will also see the marker light goes out when the signals are called for because now the bulb is powered up from both circuits but there is no ground so the bulb stays out.
the other thing I have seen over the years is the little plastic or fibre washer under the bulb, where the contacts are and the wiring comes through the bulb holder, has an index bump or slot in it so the bulb contacts will line up with the contacts in the bulb holder. if the index mark gets sheared off or disintegrates then the indexing is not the same anymore and the washer is allowed to slip or rotate so the bulb and the wiring don't line up/connect like they should. sometimes this can make both circuits power up when only one circuit is actually turned on. on some repairs I will go around and remove bulbs to ensure the washer under it is properly indexing and also the previous guy installed the correct bulb. I have seen a dual element bulb be replaced with a single element bulb by accident because the bulb was packaged wrong or in the wrong parts bin etc. it can really mess you around. if testing for a bad ground at a light turn the lights on and touch the test light probe to the bulb socket. if the test light comes on then there is no ground on the bulb socket so power is going through the bulb to the holder and powering the bulb holder.
hopefully that all made sense to somebody. lol.