I'm showing in the schematic that Red is power, White is low beam ground,
and Gray is high beam ground. So first, I'm suspicious that you're
testing the wrong wire (but I've also seen the schematics incorrect, so
you could very well be on the right track, too).
Second, before you start hunting the voltage drop, check to see whether
the bulbs are installed correctly and the lamps are aimed well. The
number one cause I've seen of complaints of dim or improperly aimed
headlamps is improper bulb installation.
As for your voltage drop, I'd suspect it's largely in the junction block.
If you've tested everything correctly and you do indeed have a 1.5V drop on
the ground circuit, I'd say that's too much. I also think anyone who says
that anything more than 1V drop on this type circuit either doesn't know
very much about automotive electricity or doesn't actually want to do the