I believe the speakers are common sizes, but the bolt patterns might not be (3 bolts instead of the common 4).Are these speakers common sizes that can be used in other cars?
One forum member reported that replacing the speakers (HK system?) caused a malfunction of some sort. Best guess is that the impedance was too far off. The best performing speakers all seem to be 8 ohm (some are 16 ohm), common car speakers are 4 ohm, while HK seems to like 2 ohm.
Be careful when selecting speakers for a car, since many good performing home sound system speakers have no consideration for water resistance in their design.