Worth noting George there is an embellishment of the generic headlamp cable colour codes you mention and particularly relevant, as I believe in this case where headlamp circuits are fused individually according to side and dip/main.
R/H main, blue/white
R/H dip, blue/red
L/H main, blue/slate
L/H dip, blue/pink
So in the OP's case it's the circuit with blue/slate cable he needs to be looking at.
Also worth mentioning to Thomas that any voltage measured at headlamp end of cable should be with bulb in circuit. Open circuit voltages measured by (high impedance) meter will likely not reflect the severity of corrosion resistance limiting current. If you measured 8 volts without bulb I can guarantee it would be a lot less with bulb present in circuit. Note my earlier comments that although fuse may be intact it could still have corroded contacts.
