Looking to redo my '92 Mercedes' stereo?

Ya, just get new speakers for your car. If you don't already have an aftermarket head-unit I would suggest getting one if you want better sound quality and features. If you don't want to change out your head-unit, just get an amplifier for your speakers. Your front speakers are 4" (in the dash) and your back speakers are 5 1/4" (rear deck.) Also, if you run an amplifier to your speakers, you will have a lot of treble and high-pitched music when you turn up your volume. I suggest getting an amplifier for you car, only if you want low-frequency notes to hit. (just a note: when you add a subwoofer(s) for a complete sound quality system, you should add up all the speakers RMS (continuous power) and multiply that by two. This number will give you rating of RMS that your subwoofer(s) should be. Good luck! E-mail me if you have any questions!