61128384473 - also has an extra connection for the trimble unit that is required for mk 1/2 nav units. it is wired so that it also supports mk 3/4
61126915132 - does not have wiring for the trimble unit so is only capable of supporting mk 3/4 units

either harness will do, you should find one on ebay or order through a dealer.

you will also need the following bits and pieces

GPS Antenna 65 90 8 375 944------------\
GPS Antenna Mount 65 90 8 360 725 ------> these 3 can usually be ordered as one on ebay, either as BMW or aftermarket
GPS Cable 61 12 8 377 434---------------/

Computer Bracket 65 90 8 361 459
Boot/trunk Panel Cover 51 47 8 190 763