[Brandish]

Have fun...

Most MGBs electrical systems are wired for reverse polarity, unless you've had a conversion done. Your yellow wire is constant 12V, or unswitched, red is switched ACC and black is of course ground. The blue wire is for either power antenna or amp turn on (or both). You probably wont need it unless the car has a power antenna, or if you're adding an amp.

Be extra careful when working with the wiring in that car... MGBs have some of the worst reputation for poor wiring from the factory, an amongst other things a lot of the circuits are not fused, so keep that in mind. Good luck.