For those anal enough to want the button backlighting to match the rest of the car, here's a brief description:
Remove the unit from the roof as described earlier. Locate the 9 torx screws and remove them (circled in green).
The front lens assembly should fall away. The buttons are loose, so be careful they don't get lost. The plastic backshell is held in place by four clips (2 at the rear and 2 on the side towards the rear). Use a scredriver to lever on one side to dis-engage one side clip. Repeat for the other side. See below for location:
Now lever the back 2 clips away until the backshell separates. This is what will be revealed:
Lift the PCB off the two plastic posts. You will now have the two PCBs connected by a ribbon cable:
De-solder the 4 x LEDs (shown circled in red), taking careful note of polarity. Solder in your new LEDs, observing the correct polarity. I used red PLCC-2 (otherwise known as 3528 or 1210) SMT LEDs. $6.99 for 20.
Here's the result. Excuse the poor photography, it's difficult taking this shot in the dark: