Thanks Dr. Fuentes for your recognition of what was one Pain In The ___ job! Although my install was complete in nature (I replaced everything, not an OE wire was used anywhere), as I understand it later W124 cars ran 4 ohm sound systems, which are MUCH more easy to upgrade.

In answer to your questions:
1.) There are several amp configurations: give the S/N to Beckers and inquire because there could be either 2 smaller amps or one large finned amp (which I've heard runs the lower freq,'s only). If you'd like, peek behind your passenger-side trunk liner, below the hinge: let us know what you find.

2.) Your dash speakers are either one way deals or dual cones. A neat feature is that the front left one runs a dual voice coil feed so it can also work as a speakerphone for cellular. It's anybody's guess where they cross, but it's fairly low methinks.

Those are not really subs in the doors, but simply 6 1/2" low-freq drivers. I doubt they go/are capable of going subsonic.

3.) Rear deck speakers are typically 2 way speakers with 5 1/2" woofers & dual tweeters pointed in different directions, for dispersion.

4.) I haven't done so, but I think you can do it with your '95; I'd definitely try in your case, just confirm you're outgoing speakers are 4 ohm (remove a dash speaker & find out).

Good luck!

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo