Headlight electrical question, bad ground Options

[vin man]

On the driver side of my 71, the low beam comes on, but just barely. High beam works fine (Both beam are good on passenger side).

I have replaced the bulb, but still have the issue. While I suspect a bad ground, if that was the case, wouldn't the high beam not work properly has well?

According to electrical diagram, there is only one ground wire coming out of the headlight.

Anyone have advice on how to troubleshoot? I've checked the ground connection at the body and it is good.

Thanks.

[vin man]

Did some testing tonight. I removed the headlight bulb from the socket on the driver side (this is the side I have the problem with).

Turned on the headlights and set to Low beam:
A- passenger side low beam works properly
B-Driver side voltage across white wire and ground is about 5.5V
C-Driver side voltage across yellow wire is about zero

Turned the high beams on:
D-passenger side high beam works properly
E-DS voltage across WHITE wire is now about 11.3 V
F-DS voltage across yellow wire is still about zero

According to my electrical drawing, the White wire is for HIGH beam and Yellow Wire is for low beam. So a little voltage is coming through the high beam side when I have the low beam on.

If I understand my drawing correctly (which is pretty scary as I couldn't read these things at all a month ago), it can't be the headlight switch or the high/low beam relay because they feed both lights at same time, not individually.

I think the next thing to check is to see if I am getting 12 volts on both sides of fuses 3 and 4 in the fuse block when the low beams are set to on.