I got an issue that is keeping me busy and driving me crazy.
Rear right woofer has became nearly silent, you can barely hear it. The midrange in the door card sounds loud as normal.
I replaced the speaker with another three spares I had laying around, with two same effect, with one there is some "movement", but again very low.
Removed the radio and replaced with another from a neighbor, same.
Open up the ISO connectors and swapped left to right the pins (a major pain in the butt), same. Is only that side that gives trouble.
I replaced the amp in the door with another I have laying around, same.
I shortened brutally the cable going through the door, in case it had gone bad somewhere unseen, no improvement.
I tested the speakers with a multimeter, all give around 3.4/3.5 ohm. The midrange in the door card, 2.5 ohm.
I tested the output to the woofer in the door amp, both give around 10Kilo ohm, I do not know if is right, but both are consistent.
I do not know what else to do ....

I am now seeing the ETM and the wiring diagram for the rear door bass speaker looks weird (page 144). Can someone help me interpret which cable goes which way in those connections? I can reroute them to match the diagram, if I can understand it (I am tired today, played with it for three hours after work ...)
Someone has messed around in this car in the past, so something might be out of line. I do not remember if this worked before, but I think it did.

Are you sure your spare speakers are good? Testing them for resistance will show if the coil is good, but not if the cone will move. Just about every one I've looked at where one woofer isn't working I've found the cone has seized so can't move. I'd start by swapping the speaker with the one in the other rear door which you know is good, just to be sure.

What's weird about the wiring? Pins 1 and 2 from the amp go directly to the speaker coil. The amp incorporates a crossover so full range going in and it is split so bass only to the woofer and mid and higher frequencies to the midrange.

I had not thought about the speaker "seized" ... it sounds extremely funny but hey everthyng can happen. Now that I think, the speakers I pulled from the bin have not been used in a while, so it could be indeed.
Wiring became clearer, especially after I noticed last night I was linking at the wrong page ... I got a Highline system. I think I will chase the "frozen" speaker theory :-)

Now the next following question would be ... how do you "unfreeze" it? 🙃
Re-cone it would be the simplest answer I can come up with ...

leolito wrote:

Now the next following question would be ... how do you "unfreeze" it? 🙃

You don't, you sling it in the bin. I've replaced all my woofers with JBL Stage 600 speakers (https://www.amazon.co.uk/stage-600CE-170mm-Component-Speakers/dp/B06XHL5R74). Fit straight in and sound way better than the originals. Even if you just get a pair, you can put them in the front and the known good ones from the front can go in the rear. You even get a pair of replacement tweeters which you could fit but I didn't bother as my old ears couldn't hear any difference.

There's two ways speaker cones become seized.
1) Constantly overdriving them, so the voice coil gets too hot & the varnish bubbles & sticks to the gap in the magnet.
2) Storing them in damp conditions for years & either the coils or magnets get corroded & stick.

Interesting, Pete. Here probably is option 2), both my car and the donor parts were unused for a good 3 years, and the spare were indeed in a more damp place. Well ... now I learned besides the mechanical, also the radio needs to work!

Richard (once again) you nailed in the head, it was exactly that. For now I mounted the one that looked more 'alive', and closed everything as I have my parents over next week on visit and it would really pain me to show ONCE MORE AGAIN the car disassembled ... they will not notice a loss of sound in one rear door 😆
I will look into replacement in a later time, when guests are gone!

I have a friend has an old man which works to round up his pension fixing old audio stuff (cassette tapes machines, old amps, speakers, and the like). I have seen some repairs on a sub and a woofer, both "proprietary" designs of German youngtimer cars, whose owners did not want to lose originality, and in both cases the results were satisfactory for a little change. Will pass by and drop the speakers to see if he wants to play ...