Unless it's a very old Bimmer, you have a factory amp.
Regardless, if you plan on running a pair of 6x9"s off the oem system with the hopes of adding an amp (or replacing the HU) later, I would go with either the DB line from PolkAudio, or the Reference Series from Infinity. Either of these are efficient enough to get a decent amount of volume from the oem amp, and are still beefy enough to welcome more power later down the road.
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