2003 SL multicontour seats: done some repairs, but no joy yet, please help!

[Danpro1]

Hi guys,

I love my 2003 SL55, bought last September!

The multicontour bladders do not work on both seats. I looked at many posts on this subject in this & other forums, and have tried to fix this, to no avail. Prior owners of my car have tried to fix it, because I found poorly done repairs, so the solution might not be a trivial one. Here's what I have checked and done so far:
1) Removed both seat backs and inspected visually: nothing seems broken.
2) Removed both seats completely and inspected: fixed two broken white tubes (one of them leading from floor to seat).
3) Tested each seat by blowing air into entre tube: both seem airtight.
4) All relays on both seats work, because I can hear them click from the back of the seat for every push of any of the multicontour buttons.
5) With car running, pressing any of the multicontour button does *not* make the pump in the trunk start, although the pump works fine for door locks.
6) Same behaviour with engine running, or key in #2 ingnition position.
7) I have pulled and put back the fuse #71 twice to reset the PSE pump, without a change in behaviour.
8) Each time I try the "Pulse" switch, on either seats, I get a P1555-032 error code (cushion seat - lower waist cushion). I DO NOT get error codes with any of the other multicontour buttons, on either seats, still none of the seats bladders inflate.
9) Bought an iCarsoft i980 code scanner to reset seat error codes after each test.
10) The entry "Y" connector on both seats had one end open (circle in pic #1), so I capped them both.

Here are my questions:
a) I am wondering about a white tube that is *not* connected in each seat (circle in pic#2) and I am wondering if this tube should be connected to the entry "Y" connectors that was missing a cap? Or is this tube required only for a function that I do not have on my seats (I do not have "dynamic" or ventilated seats)?
b) What other test can I do to isolate the problem?

Thank you for your help,

Daniel