If your speaker grill is too far gone, an unfinished replacement is available from Bud's Benz for $80. You will need to sand it with a 220 fine sandpaper to smooth it properly and remove any traces of glue. It is solid mahogany, not veneer, so it is great to work with.
Stain the piece to match the other pieces in your car. Then, as Rick says, use several light spray coats of Helmsman Marine Varnish. It takes some time, but it is not difficult. The important thing to a great finish is good prep, a dust free environment with low humidity, very light sanding between coats. Take your time and don't rush the job.
Best regards,
Don
Cleveland, OH
71 280sl