Lighting problem- where are the earths?

Driving home today I've discovered an intermittent earth fault. With the headlights off, everything appears normal, but with the headlights on, the front left lights are misbehaving. The headlight itself is dim, and the indicator doesn't work on that side. The left rear indicator is also constantly glowing dimly, along with the dashboard indicator and main beam warning lights. So I suspect there's a loose connection somewhere in the front left lighting loom, and when the headlight is on, the voltage drop is causing the current to return via these alternative paths. There's presumably not enough voltage across the flasher relay to make it trigger.

So, how are the earth wires laid out? Presumably the earths for the indicator, side light and headlight all join together somewhere, and the fault will be after that as they are all affected. From here they should logically go as a single wire to a point on the chassis or another earth wire downstream of the problem. My first guess is that the fault lies somewhere after the junction between the lights, and before it joins any other circuit.

I would just work my way around the earths on the individual lights first, clean up and secure