Greg, here is the info on the car's main harnesses - 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 - youare prewired for all the lower level options, even if they are not installed...

this is only confirmed so far with heated seats and with your own statement about 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