Look at the VIN for you car. According to the MGB shop manual, aka BMC
service manual, if you car starts out GHN4/GHD4 or later, it started out
life as negative ground, earlier cars were positive ground. Basically
anything after the introduction of the Mrk.II model in around
November/December 1967 you're looking at negative ground/earth.
If your Haynes says otherwise, then you've found one of the not too few
errors in Haynes. Haynes is useful, and has some clarifying pictures, but
don't take it as gospel.
From: Rick Sinclair <RHSinclair@USWest.net>
To: MG list <email@example.com>
Date: Thursday, December 16, 1999 5:03 PM
Subject: Electrical Diagram
>O.K., who switched my manual on me? Went out to try and fix some of
>the electrical stuff on the 'B, and almost everything I look at in the
>wiring diagram in my manual is different. My car is a negative ground
>(earth) car, and all the haynes diagrams are pos. ground. Is this
>another case of the dreaded DPO syndrome? Does someone know where I
>can find the correct diagram? Your help is appreciated.
>Rory Sinclair <:-P
>P.S. Yes, it is the right manual.