I did this retrofit last year. The mistake I made was buying a rear speaker and front and rear module. You need a rear only module from a E46. I can't remember which wires I switched around but I switched a couple of the wires around in the black connector to make the beeping go off. The rear only system actually uses the front speaker by the pedals for the alert sound. The button panel is not necessary as the system turns on and off when switching into and out of reverse, but I added it for a manual override. I'll try to take a picture of my harness so you can replicate it.

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2004 X5 3.0 6 Speed MT
Build Date - 09/03
156K Alpine White / Black
Premium & Cold Weather Packs, Nav,
Tons of lighting mods, 4.8is Exhaust, Headers, Eibach Springs, OEM 87's, Custom DME Tune, OEM Hitch.

2005 X3 3.0i - 100K - Blue water Metallic / Black

2005 X5 3.0i - 140K - Alpine White / Hellbeige 2