Sometimes it's best to start at a connector and work your way through the wiring harness. In the case of the ground you circled in red, unplug the front combination connection and probe the ground to see if it is a ground. Check for continuity between connectors along a wire's path. At some point there will be no continuity and you'll know at least what wire segment is at fault.
You might find that some of your grounds are not grounded due to rust.
I ended up having numerous corroded connectors (get some terminal cleaner), melted wires, wire splices from some long forgotten accessory a PO had installed/removed. In the end, I think I've removed and rewrapped every wiring harness except for those in the engine compartment. I also have a spare dash and cabin harness hanging in my garage that I sometimes use to trace wires.
If it hasn't been done already on your car, you might want to upgrade the headlight circuit (http://www.zhome.com/ZCMnL/HeadlightRelays/JudkinsRelay.htm)