Ian,
My local Radio Shack no longer sells car audio. They sell the RCA mini/cube speakers for use with a home compact stereo system. They were availible, as of last summer, in black or white.

Putting speakers in the side kick panels, as Scott suggested, is something I did with another Mercedes many years ago. It's probably your best choice for decent sound, if you're willing to cut large holes in your kick panels.

If you're really fussy about cutting your original kick panels, you could remove and save them, then fabricate new panels for the speakers.

I'm testing some different speaker systems in my '60 220S, that don't require cutting holes in the interior. I'll let you know how it works out.

Happy Motoring, Mark

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DrDKW