Delco CDR500(E) Vauxhall. Not working?

Hi. Bought a vectra with a Delco CDR500(E) head unit. First i noticed was the detachable front panel wouldn't stay on one side so put some blue tack on to secure it. When the ignition is turned on the light behind the volume control button comes on code isn't asked for on display unit. Have the 4 number security code on the unit but don't know how to enter it as no manuals. Am i correct in thinking i have to press nos. 3 & 5 to enter the code? Do i need a new front panel or could the fuse need replacing? So many questions, so many answers!! Cheers, Andy