Right so I found it abit tricky trying to buy the right amp bypass lead to fit an aftermarket head unit for my tezza, but I managed to get the right one in the so I thiught I'd post it up here incase anyone else has the same problem in the future.
http://www.ebay.co.uk/itm/PC2-92-4-Lexu ... 6016.l4276
It's a simple plug and play item which makes it easy as s@3! to install, the only thing you need to do is wire the red wire from the head unit end to a switched live or permanent live like I did with the cigarette lighter
Hopefully this will help someone in the future as all the information I found on it had outdated links on where to find the part. If anyone has any questions about it give me a shout and I can walk you through it