Wiring / Earth Question

I have a Li 150 Special. The headlamp adjustment is by a screw which moves the lamp to a different angle. From the screw is a cable which I assume it to an earth. Is this correct?
Next question. The wiring diagram shows the speedo light connected to the wiring block, fine. but also shows an earth connection. How is this achieved? As the speedo is plastic and I cannot see how it is therefore earthed.
Any advise please.
Rob
Next question. The wiring diagram shows the speedo light connected to the wiring block, fine. but also shows an earth connection. How is this achieved? As the speedo is plastic and I cannot see how it is therefore earthed.
Any advise please.
Rob