Space between rear cargo area and the engine

If you crawl underneath you should see space between the front firewall of the engine compartment and the back wall of the passenger cabin. In my older Intermechanica there's about a foot of free space directly above the differential area of the transaxle. It tapers a little as it goes up, as the back passenger cabin wall leans back. The firewall should have a 6 or 7" round hole for air access to the engine compartment. 

Others have used space on either side just to rear of seats - where the top hinges mount.  Those areas are NOT sealed but you could easily fiberglass in boxes or pods to protect speakers. Plenty of room for 6" round or 6x9". Check depth on sides though.