Not sure what the metal rectangle is.
This MAY work:
http://www.pac-audio.com/productDeta...&CategoryID=29
Since E89 Z4s are so low volume, I find a lot of products don't list it as supported. However the radio subsystem appears to be identical to E9x 3 series, so what fits a E90 should also fit in the Z4. At least this is what I found in installing a GROM MST3 (to add USB input) on my car, which is non-idrive and has the base audio system.
Note that by using an aftermarket radio, you will lose stuff like (if equipped) sat radio, bmw assist, bluetooth, pdc, and the gong sound.