You can hook up a head unit without replacing anything else, I have done this in my 91 500SL. The speakers are also easy to replace, dont get misled by "experts" who tell you that speakers have to be custom made, just stick with a low profile basket like JL or MB Quart. MB Quarts fit in my stock 500SL boxes with only very very minor modification to fit the speaker connectors.

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2007 Corvette Convertible Torch Red
1996 MBz SL500
2008 Harley Davidson Night Rod Special