This is an easy fix.
All you have to do is remove the vacuum pipe from the back box valves, 1 on each back box. They just pull off, very easily.
You will need to remove the under tray to access it (if you have small hands, you may be able to reach in past the exhaust, but will not be able to cap it), and make sure you cap off the naked vacuum lines, you may wish to re instate it at a later date. If you were to put a suitable bolt in the end and use a cable tie round the top of the pipe so it grips. Then just cable tie it up out of the way, jobs a good'n and all for the cost of a packed of cable ties.
Incidentally, the solenoid that controls the vacuum to the rear boxes is situated underneath, just in front of the nearside/passenger rear wheel, under a black plastic cover. You could again, disconnect the pipework here, in a similar manner to the method above, then you don't even have to take the under tray off.
..or even put an on/off switch on one the 12v supply wire, then you can just turn it on and off by simply reaching underneath....or to go the whole hog, take it to an auto electrician and get the on off switch put inside the cabin, but this turns a very simple job, into a fiddly and costly one.
