When I brake my nearside brake light is very dim and when my lights are turned on it actually goes out when braking?

Most likely an earthing problem. Check and clean your earths!

Where are they?

Rear light cluster earths, very typical

I havent a clue where to find the earth, I'll check my Haynes manual...

Check this out:

http://www.volvo300mania.com/forum-uk/v ... =39&t=4384

You probably don't have to go to the lengths Chris did by running new earths, but a good clean of them should sort it.

Miraculously I replaced the number plate bulb that was dead and swapped the connections around on the brake light that wasnt working and voila, the entire rear lighting system works.

Just out of curiosity where are the rear light cluster earths?

It's the aluminium looking strip that all the bulb holders try to rub against. Normally as a temp solution some contact cleaner on both the copper and the ally sorts it out for 12months or so.

I'll try and sort out the pictures on that thread in the next few days

sometimes just disturbing the earth strip will scrape off some oxidised aluminium and restore good contact... ensure your lights aren't flickering over each bump.