I've done the EBay and on-line route with many speakers in Mercedes over the years. Last time I needed replacements, I just removed one of the old ones and did some shopping locally. I ended up buying a pair off the demo rack. The sales guy was very helpful, he even let me take one out to the car and do a 'dry fit' so I could be certain they'd fit. The specs for the speakers you want can be researched on-line, so you know what you want, but nothing beats holding the prospective unit in one hand and the old one in the other for fitment.

If someone here has done a replacement on the exact same car as yours, you could look for speakers like they used, but they may or may not be available to you. Those speakers look just like the old ones in my '85 380SE BTW, which I replaced with Eclipse units off EBay (sorry, the job was years ago and I don't know the numbers.) I paid a lot, but they sound fantastic (and they were used, so I was very lucky)

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1986 560SL
2002 Toyota Camry
1993 Lexus