Help! Radio won't power up!

Hi,
I've just bought a Gamma Radio for my golf mk4, but it won't power up. Normally, these radios ccome on in safe mode, but theres no display. It lights up red when the headlights are on, but thats it.
The radio was bought off Ebay with a code from a legit seller & it was fitted to his passat. He took it out working & sold it to me.
I've since read on various forums of people doing simialr things & getting the same result (ie Golf to Passat/Passat to Golf). Is there a shutdown security feature in these radios to prevent use, or is it buggered? BTW, power is there to the plug & fuse is OK.