headlight ground location?

[mark12]

My drivers side headlight is very dull and obviously not putting out full power. Ive tried the simple things like swapping globes etc but it hasnt worked.

So I guess that means its most likely a bad earth then?
My problem is that I have no idea where the drivers side is even earthed too

Im having trouble tracing it back haha, would it be on this diagram or in the manual anywhere?
sorry if this is a noob question!