trailer hitch - ONLY BMW original, nothing else!!! with dealer's labor - about $1000.

steering wheel - it was done, you need the wheel, a special contact ring, and i think a fuse to complete the link...

for the seats, you will need the seat and the back (from a wrecked one) or figure how to stick the aftermarket heating pads into your seats...
you will need the 61316988957, the switch, about $70 from a dealer, you need two...

i am checking to see if you need a relay or whatnot...

to see if you are really prewired, you might want to yank the rear seat up (it is pressure fitted, no screws, no bolts)... just place your hand on the edge of the seat, approx "between" the legs of the rear right passenger, and yank it hard up... then, place your hand on the edge of the cushion "between" the legs of the left passenger, and do the same thing... the middle seatbelt does not come off (needs to be unscrewed, no need now), so you would need to pull it and feed through the hole opening of the seat as you move the seat away from the base - the seat will not be completely out but you can stick you hands around to see if there are any wires/connectors that are hurled away, taped or hidden... if you find them, your chances of successfull retrofit are increasing... lol....