Two of the ISO connectors on my R129 are the standard 8-pole varieties (one for power and the other for speakers), and there's a third, 10-pole one that came from the memory that I assume supplied power to the optional amplifier—which, to be honest, isn't in my vehicle. Even though my car only has four wires connected to the 8-pole ISO speaker connector, the VIN decoder—which I have used a few times—suggests that it has rear speakers. In this thread you can find various images of these connectors.
There are two tweeters in the dash, a larger (bass) speaker, and a smaller (midrange) speaker on the R129 front doors. Being on my second R129, I can say that the original speakers are mediocre at best. I replaced them with JBL units of the same size in my previous vehicle and am planning to do the same in my current one.
The car's factory stereo was a Sony cassette player with a few shaky functions; I'm replacing it with a higher-quality CD player, which doesn't have any built-in amplifier. I plan to install my digital signal processor and multi-channel amplifier in the trunk, with the original speakers mounted on the sides of the back seats and a passive subwoofer on the luggage shelf—which forms when the back seats fold down—that can be easily removed if more room is required for luggage.
It seems like someone may have tampered with the wiring in your car, since the front door speakers and dash tweeters are connected in parallel. I'll be installing a 6-1/2" bass only unit in the lower door positions and 4" dual concentric units in the upper door position, also wired in parallel. My previous car had 5-1/2" dual concentrics in the rear, but this time I'll be using a 5-1/2" component system with the bass unit in the same spot and surface-mounted tweeters higher up for better sound dispersion. I'll post pictures of these in my thread once I have better lighting.
Typically, the speaker outputs on the main radio are where the Becker amplifiers are connected. When I acquired my W140, it was already equipped with the original Blaupunkt unit and a factory amplifier in the boot.