Thanks to Jamoir for his work on this thread - I was trying to get the boot lights in my 159 SW to work, and having read this I was happy to get the spanners out and try my luck!
Looking at Jamoir's photos, it looks to me like the connector to the boot lock are different in the Brera and 159 SW - so this update might help some 159 owners (no idea whether the saloon will be anything like this).
The problem as mentioned above is that there are two microswitches to monitor when the boot is open - one is used to bring up the 'boot open' warning on the dashboard (and probably for the alarm) and the second operates the boot lights.
Why two switches are needed when one would do is beyond me.
So, it appears one of the switches fails, and hence so many of us have no light in the boot - the solution (as suggested by others) is to piggy-back the signal from one switch into both circuits.
This took me about 45 minutes, though mostly that was faffing about to remove the boot trim - it's the most awkward part of the job, and also where we start.
So, to remove the boot trim...
This involves prising out the two openings used to access the boot-mounted lights, along with a handful of smaller openings, and undo a total of 10 hex-socket bolts, using a long allen key or hex driver. I can't remember the size, but it'll be in your socket set. Do yourself a favour and line up the smaller plastic covers, I think they're all unique in shape, and it'll save trial and error when you re-fit them later. I think there are five in total, but you can't miss them.
Now prise out the red & white lamp from the passenger side of the boot lid, unclip the lead and set aside.
After wiggling the loose trim piece you'll discover that the trim is held on by a further four brackets (bit of overkill there?) which are chuffing fiddly to release. In theory you pull a grey plug out of each one, but I lost patience and pulled down hard, breaking one of the mounts. I'd advise you take this bit slowly and try not to break anything, but it's your car!
Now you should be able to access the lead that clips into the boot lock - there's only one, it's white and has five wires.
Unclip this, and with a bit of experimenting with your multimeter, you'll discover that the blue and yellow wires are the ones you're interested in. It seems the blue wire connects to the defunct microswitch, and the yellow wire connects to the good microswitch.
Essentially, the feed from the yellow wire becomes 12v+ when the boot is opened, so you want to feed this into the blue wire heading off into the loom - that way your boot light comes on.
Now, you can use a Scotch clip or solder if you like, but I used a chocolate block and a short (in this case, grey) jumper wire.
You don't need to mess with the red, black or grey wires, I assume these are 12v+, 12v- and signal to open the boot.
Now you'll want to test out your handiwork, and note that if the boot has been open for a while to do this job, you may need to close and re-open it to reset the lights (I think the car turns off the boot lights after a while to save the battery if you forget to close the boot).
If all works properly, tape up your wiring fix, and refit the trim.
Hope this helps anyone still fumbling in the dark, cheers to previous posters for finding this fix in the first place!