all E53 are prewired with Level 1, it is a "plain Jane" model. then, once you have one option from Level 2, all options in Level 2 are pre-wired...
If you have one option from Level 3 - all options in Level 3 AND Level 2 are pre-wired...

So, if you have the Level 5 - you are prewired for all the lower level options, even if they are not installed...

this is only confirmed so far with heated seats and the PDC.

Level 2
S235 - trailer hitch
S502 - SRA (whatever it is)
S508 - PDC
S609 - Nav
S624 through S629 - telephone/radio

Level 3
S248 - steering wheel heater
S261 - rear airbags (interesting, I wonder if this is retrofittable)
S265 - RDC, tire pressure
S461 - backrest heating system in the rear
S496 - seat heating system in the rear
S533 - rear air conditioner
S536 - auxialliary heating system

Level 4
S220 - single axle level control

Level 5
S522 - xenon lights