6v bulbs

Hi All,
I have just purchased my first st70 and it is a 1979 version running on a 6v electrical system. I am busy soon the little bits required to get the MOT and one of these issues is the rear bulb. I bought what I thought was the right bulb a 6v 21/5w, but only the break light, not the rear light. Does it need a 6v 18/5w or will this make no difference?

Any help is very much appreciated.

Regards
Ben

So the rear light doesn't work? But your brake light does work?

You should check the wiring first.
The brake light uses the battery, the rear light takes the electricity from the coil.

So check:
- coil
- wiring
- regulator
- connections inside the rear light (oxidated, rusty,...)
- bulb (yes, even when it's a brand new one)

Both the 21/5w and 18/5w should work (if the correct connection type)

Also, the brake light works but only when the light switch is turned on. Wiring is not my strong point, so this type of thing proves confusing to me.

Are you sure you just didn't switched the connection for brake and rear light?
(Check the matching colors right behind the rear light bracket)
If not, you need a multimeter to check the wiring.

Good luck!