They make 5.25" aftermarket speakers but you have to go hunting for them as most car/auto accessory shops don't carry them. I had a pair of Rockford Fosgate Punch 5.25"s but you don't need to go spending big coin (although those speakers had a killer sound to them - nice and warm due to the cones being paper fibre) And the 2" speakers in the doors? Those aren't just plastic grilles concealing the tweeters mounted to the door chassis? Look on ebay for the 5.25"s - there are plenty to pick from and some good brands up for cheaps. What speakers are already up on the back parcel shelf in the beemer? If you can get a dual cone speaker that will swap in without grief I'd go down that path (it will make life easier on you) And when buying a speaker - if you can get your hands on the set you're looking for check out the size of the magnets, the quality of the speaker chassis, the speaker wire connections and how flexible the speaker cones are - these are all indicators of their quality. E.g. - thin, pressed steel chassis, small magnet, speaker cone feels like a disposable plastic party cup and a nasty looking mount for the wires held on with a tiny rivet? You can bet it will be junk...