recently installed a rockwood 10" sub in the boot of my Nc Lane running off the factory alpine 3 channel amp. the amp is putting out 100w rms to the 200w rms sub, but still handles it well. I decided to mount the sub behind the rear seat on 12mm ply to conceal it and also seeing there is a gas tank in the boot, keeping space in the boot. with the gas tank behind the sub it almost makes a sealed enclosure of about 25mm deepX70mm highX1200 wide. The sub is also a free air sub. thats the history out of the way......
now the sub works well and goes through the front seat at high volume...... but at relatively low its only felt in the back seat. I have ported the original 8"factory sub hole in the parcel, but was wondering if running a port through the back seat behind the arm rest would be worth while
( back of the back seat I think has sound deadening padding built into seat). I am not a big fan of doof doof music, but respect a subs place in sound quality

i had the exect same problem, i was running a 200wrms kenwood 10" sub at 150wrms from the amp

after alot of stuffing around, i realised a 10" sub does not have the power to penetrate into the cabin of a falcon, with a big boot and chunky back seat.

Adding to the problem in your case is that its only running at half power....

your better off getting a low powered 12", or amping the 10" to its potential.

im now running a 150wrms 12" and pumps 100 times better than the 200wrms 10" i had before