I'm not sure about e46's but on e36's there is no "antenna"
There is a secondary matrix on the rear windshield. Looks just like a defroster matrix, but its sperate and serves as an antenna.
The signal from this antenna is sent through a preamp and signal booster prior to being sent to the stock deck which must also boost the signal.
Aftermarket decks in e36's I have worked on are NEVER nearly as good as the stock deck for FM reception.
But what do I know... I hate radio... I removed my antenna and welded up the hole