88 should mean you have a Grand Prix model 780, which gives you a short list of options...

The 780 is not capable of controlling a CD changer, so you'd have to go with one of the CD changers that uses FM modulation (you listen to the CD changer over your Grand Prix's FM tuner). Your sound quality will suffer, but if you're not picky it lets you keep the stock appearance of your Becker.

One exception - JVC makes (or made, not sure if you can still buy them new, but they're not too hard to find surplus on the Internet) a model called KD-MA1, which splices into your car's wiring, thus improving the sound vs. one of the FM modulation models, and lets you keep your Becker.

Other than that, you're going to want to replace the Becker. Many people go with one of Blaupunkt or Nakamichi's more conservative-looking models that don't look *too* out of place in your Benz's conservative interior (as opposed to the 30 blinking lights in 16 different colors you find on lots of decks these days).

You'll lose the use of the fader control by your gearshift and may lose the speed-sensitive volume, but otherwise your sound quality will improve, even without speaker upgrades.