Remember you need to do a few things: (A) select a head unit that meets your needs, (B) physically mount the unit, (C) wire it to the car and (D) provide a steering wheel interface to the non-BMW head unit.

A. Your head unit (double DIN) will be a personnal choice, I plan on buying the JVC (not Kenwood) DB60ATE based on the specifications / functions in my earlier post.

B. For the head unit mouting bracket and bezel, there is a guy on Ebay.de (naturally in Germany) who sells a VERY good product (better than Metra) for a very reasonable price. His seller ID is striker1980. He's not a cousin, brother-in-law or similar.

C. For the wiring adapter (ISO adapter in Europe), you'll find them easily (ebay or similar). Crutchfield for US people. Depending on the year of your E53, you would a specific one (BMW used one with round pins, another with flat pins and a third one (from memory) as well.

D. For the steering wheel control adapter, Axxess ASWC-1. I've found good reports that I works easily with the E53 CAN BUS and many brands of head units.

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2002 Euro X5 (E53) 3.0 diesel (M57)
5 speed manual with UUC short shifter.
Pre-xDrive NV125 transfer case (chain & gears).
Factory Xenon headlights, Spyder LED tail lights
Updated vortex crankcase breather.
Motorsport dead pedal, factory fire extinguisher, factory first aid kit, factory F&R mud flaps, factory PDC

Bilstein B6 shocks F&R
Redline oil has replaced "lifetime" fluids (F&R diff, manual transmission, transfer case and P/S).
Dimple magnetic drain plugs in all compartments.