Vauxhall cd30 stereo display not working right?

I have purchased a secondhand cd30 stereo to replace the cc20 stereo i have in my vauxhall combo van, i didn't have to enter a code. the stereo works and on the display when it's first turned on it shows radio station or cd track but after about 10 seconds it shows "display safe" permanently, you can still change station and cd track you just don't know what too as the display doesn't change.....help appreciated