I've got this left over from a recent bootlid microswitch repair on my facelifted E46 coupe. Its the plastic trim that houses the microswitch and covers the reg plate lights. Its a genuine BMW OEM part in primer (part # 51 13 7 170 967).

Just below the BMW bootlid badge here:


It is the primered trim only, and not the reg plate lights or microswitch: http://bmwfans.info/parts-catalog/51137170967/

€25