How to unlock radio ??

[Morja]

I have a 1990 SE Bonneville (in Finland) with the original Delco radio. Problem is that it is locked (text: loc) and any of the instructions I find from the net doesent work ?? Some say to hold buttons 1 & 4 for ten secs but the radio doesent have any numbers ?? Wich buttons should I use to get the so colled six-digit codes ? Either none of the buttons on steeringwheel does not work.