Originally Posted by Ricky Bobby

Mike - I haven't a picture of it out of the speaker grille, but it looks like overall diameter is 65mm so 2" speaker should work -

I am thinking about these for the mid-ranges on the rear door, OR perhaps just dremeling off the entire outer ring of the Memphis Audios to make it a 2.5" diameter speaker -

Peerless by Tymphany NE65W-04 2" Full Range Woofer

Here is a pic of the Factory midrange speaker, which as I said, is the same speaker whether DSP or Hifi, on the E46 as well - I think it needs to be around 2" or around 65mm overall diameter to fit into the factory "grille" which is clipped into the rear door panel -

BMW E46 3-Series Mid Range 2" Rear Door Speaker Sedan Coupe Wagon 1999-2006 OEM | eBay

2000-2006 BMW E53 X5 REAR DOOR MID RANGE HIFI LOUD SPEAKER BEIGE OEM 65138374750 | eBay

EDIT: Now that I'm looking at it, the OEM connection does NOT have a capacitor in line on the rear door midrange, you would know better if putting an additional tweeter set in that location would be good or bad -