Been reading this old thread with interest. I have a similar but additional issue for which I hope someone has a good solution.
I use my 159 SW for the summer holidays and now that the kids are a little older, have to charge satnav, iPhone, and DVD player with 2 screens. The attempted solution last year was an extension lead from the 12v socket in the boot to under the front passenger seat then another short extension to a cigarette charger socket (for the DVD) and 2 USB for the satnav and iPhone. I used the B&M socket for music with a USB stick (south of France requires a lot of music to keep sane!).
Main problem was that the 12v socket in the boot kept spitting out the extension lead, despite putting some rubber wideners on it. I got so annoyed with it that I gaffer taped it on (and it would still eventually pop out) and was a pain to clean off the tape residue.
Planning ahead and trying to find a better option:
A) change extension cord out of boot but various plugs seem to have the same problem of being spit out; or
B) have actual 12v plug in boot replaced;
C) Use B&M port but will that power all the devices and any suggestions on the adapter?;
D) any other options??