The BMW tweeter is attached to the door, while the enclosure peels back. It looks like a cheap poly dome.

The tweeter is marked "BMW", with a rating of 8 ohms (not 4!). The plastic frame that holds it has a 1.00" diameter cup, but could be modified (cut) to hold a tweeter driver with 1.25" max diameter. The depth behind the stock plastic frame is limited at one corner to 0.50", though the greater chamber has enough volume to hold a significantly deeper tweeter.

The panel-mounted midrange speaker is also marked "BMW", with the same 8 ohm rating as the tweeter. It's a foam-suspended paper cone.

This midrange has 2.75" OD, though it looks like the plastic ring and mount could easily be pulled off, leaving a large flat space to mount anything else. The depth of the stock BMW driver is 1.25", while it seems the door panel could accomodate more space if needed.

The door's bass speaker is marked "BMW" / "HAES", with a rating of 2 ohms. This is a plain-paper, foam-rolled, plastic-framed driver. The OD is 6.25", and this is a hard maximum imposed by the 3 plastic panel pegs - You really shouldn't try to use anything bigger for this woofer. The depth of this current driver is 2.75".

This mid-bass speaker is the same, plain-paper / foam driver from the front door panel (6.25" OD, 2 ohms). Underneath, there's a recycled composite enclosure that you can easily remove for a bit more space. An open hole is cut into the trunk, though it's not big enough for a deep magnet. The hard max depth is 3.00", imposed by the sheet metal.