Just finished installing new system in my 92 300TE. I installed a Pioneer P6400 head unit and Pioneer XM receiver. I used the factory speakers and bypassed the fader and amp. Wired the front and rear door speaker directly to the head unit and wired the front door bass speakers to a new 200 watt amp.
The XM receiver fit easily under the rear seat and the smallish amp is mounted under the dash. The hardest part of the install was finding the correct wires for the speakers. The front speakers was easy as they are connected behind the dash with plugs. Front and rear door speakers were a bit challenging.
The XM radio is incredible. The digital sound really brings the system to life. Even with the factory speakers, I can get high clarity and very good speaker separation. The head unit allows an almost limitless amount of adjustments to sound quality. I can get surround sound effect with the unit.