To minimize hacking up your interior, I'd suggest adding a single matching speaker to the center of the rear shelf, behind the back seat. My 250C has the factory Blaupunkt stereo radio with a 4 X 7 speaker in the dash and a matching 4 X 7 speaker in the rear. It sounds much better than the 4 speaker Becker setup that came in my '72 250 sedan. If you still have the cardboard panels under the dash, you can mount a pair of modern speakers there like I did in my sedan.

Happy Motoring, Mark

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DrDKW