I normally do older stuff but I have serviced some Beolab Penta's and if I remember correctly there is a cover held by a single screw in the connector bay.
Usually, there's a reason for a fuse to blow but I've had more than one case where a fuse was actually the only faulty component in a Beolab Penta. There are two fuses in each speaker, one for the amplifier section and one for the electronics (link control etc.). Make sure you replace with the same type and rating of fuse. In your case, with no light at all, the fuse for the eclectronics would be the most interesting one.
Of course there can be other causes but checking the fuses would be the place to begin.
Martin