Nuevo high level brake light

We recently picked up our 2004 Nuevo and are making progress with a few snags found after inspection. 
We have fixed the trumatic on gas (bent control rod, air in pipe and cover missing a screw so didn’t attach properly)
Number plate lights (bulbs and a bit of sandpaper)
Speakers (literally falling apart, replaced with pioneer 3 way coaxial for £50)
Aerial (in the Luton and you need an arm that articulates in three separate directions)

Today’s project was the high level brake light. Full of rusty water and totally unrecoverable). I managed to recover the leads after a connection gave way and disappeared) and before sikaflexing back in place wondered why there are three leads, two black and one red, rather than just two?

I had to do the same when I had a Nuevo (also 2004) - was full of water, so just replaced with a Chinese LED one off Amazon that was a tad smaller but a lot cheaper and just sikaflexed back in no problem.

It's a long time ago so not 100% sure, but the two blacks are possibly an incoming and outgoing earth i.e. the earth comes from somewhere, into the brake light and then continues somewhere else with the second wire.

To be sure - I would simply get a voltmeter on it.

Put the brakes on and measure across each black to the red and see if you have +12V.  If both do produce +12v then they are both earths, if only one does the other could be the outgoing earth.

If you have continuity with the blacks it could simply be 2 earths.

To keep things simple though, just wire back up as you found it and it should be the safest bet it's correct.

A second  earth cable  could  have been  to fix a bad earth