cabin gain

I guess it would be sort of like weighing an approximately 10 pound box on a bathroom scale;  the weight of the box alone wouldn't make the scale move enough for an accurate measurement, so you weigh yourself, then weigh yourself holding the box.  And calculate the difference.  

Measure SPL of the enclosed woofers in an anechoic chamber.  Since cabin gain is affected by all the drivers used in the sound system, you will have to fashion enclosures meeting the same cubic feet airspace as the doors (if those are the mounting locations), and include the tweeters in the mix as well.  Use the exact amount of power in the test that will be applied in the car later.  After getting a reading of SPL, install the system back into the car and apply the same power as used in the test.  Measure the SPL.  The difference is cabin gain.

Not too easy, huh?