SIII - New Rear Lights - Three wires not two?

Gents,

Bought some replacement Tail lights for my SIII SWB,

The old lights only had two wires (and earthed through the light fitting) the new ones are plastic and have three wires,

Can some one shed some light as to how to wire these?

on the fitting I have Red/Black - which I have connected to red/black
on the fitting I have Green/purple - which I have connected to Green/purple
left over is a Black, which i have earthed to no success,

the lights come on with the headlights, but there is no /very little change then i press the brake pedal,

could it just be a poor earth on the third wire (black?)
if the black is not earthed ,the lamps go out when i press the brakes,

help!

Earth the third [black] wire to the body or clean point on the chassis. Your present connection is not good enough.

I ran and earth from the battery to the dash and from the dash to the rear and connected that to the same screws as the lamp earth. With a bolt together body and separate chassis you can't assume one part of the body is electricaly connected to any other part. You have selant in the joins and corrorsion on the bolts and screws.

I ran all of my light earths to a single bolt through the floor of the tub behind the light box. I then fitted and earth strap between this bolt and the chassis bolt holding the tub in place so there is direct connection to the chassis which is then earthed direct to the battery. I also made sure that all of these bolts were clean and had a good contact before covering them in grease once tightened up to keep them clean and corrosion free.

As an aside the new plastic light fittings should also have a small metal strip that goes to one of the securing screws so it is earthed through the bodywork via this screw. However this usually does not provide enough to earth the light unit completely.