I had an aftermarket 7" in-dash pioneer system put in. He didn't have to replace the speakers, but it did cost a bit (aside from the stereo unit). I had the fiber optic system, the sub in the back is useless now.
All W203's have fiber optic - it's used for the CD changer, TeleAid and cell phone interfaces. But only if you have the optional Bose
system does you car also use that fiber network to send audio output from the factory head unit to the Bose amp in the trunk. And in that case, it does not have any analog audio wiring in the dash.
To install an aftermarket system, the installer will have to run wires from the dash to the trunk where the Bose amp is then bypassed. If using the new head unit's built-in amp, then you run speaker-level wires and tie them directly to the speaker wires coming out of the Bose amp (disconnecting them from the Bose amp, of course). If using a new external amp, then most installers will put it in place of the Bose amp, and run low-level wire from the new head unit to the new amp in the trunk. The new amp's speaker outputs are tied then to the speaker output wiring from the removed Bose amp.
The problem with the Bose speakers is that they are 2ohm impedance which is usually not compatible with aftermarket systems; it will cause their amps to overheat and fail. Not only that, the Bose speakers generally sound terrible without using the proprietary equalization circuits built-in to their amp.
If you did not have the Bose system, then the amp was built-in to the factory radio head unit and there are high-level speaker level wires in the dash harness (direct wiring to the speakers). The factory speakers are fairly decent 4ohm models (although prone to dry rot). There are built-in bass blockers for the sail panel tweeters and bandpass filters for the door speakers, routing most of the bass to a passive subwoofer in the rear hat shelf.
In my kid's C240, I installed a Pioneer Avic 7010 model using it's built-in 25w x 4 amp to power the door speakers. Originally, that was the factory non-Bose speakers, and they sounded great, but dry rot won out. I replaced them with Pioneer mid-grade speakers and we really lost a lot of bass response compared to the factory models. So I added a 12" Pioneer subwoofer driven by a 500W RMS/1000w max Pyramid amp in the trunk. I had to run one low-level speaker wire from the head unit to the trunk to use the built-in subwoofer crossover of the Avic. It sounds great, and the kid is happy (though sometimes making too much boom-boom coming through the neighborhood). Total investment is still under $1,000.