check out link below
http://www.billswebspace.com/MZ4SubwooferInstallation.htm
very neat install and fantastic info. personally, i'd prefer having the sub and amp behind seat in cabin if there's space between seats and rear bulkhead. probably dead space anyways.
The 10 inch JL sub used requires about 4.5 inch mounting depth but a shallow sub like earthquake sws 8 inch takes up less than half and works in e90 as underseat replacement subs. The new/current gen class D sub amps are incredibly small, e.g. the rockford punch PBR dimension is 6-3/4"L x 4-1/4"W x 1-1/2"H.
the one size fits all off the shelf active sub not my cup of tea cos their performance leaves a lot to be desired. and also i like a more 'integrated' finish.