Face your subs
forward and listen, then turn them around so they face the back of the trunk and I can guarantee it will sound way better.
This doesn't always work to one's advantage. Generally, the reflection off the rear of the car improves sub-bass response by adding to the wave length and taking advantage of the wave reflections. Unfortunately, in some cases, the reflection can result in sub bass cancellation at certain frequencies, which will leave a huge hole in the sound. In other words, results have been known to vary with this method and it really depends on placement and how much reflection one encounters prior to the sound reaching their ears.
I had better results with the direct cabin fire method through the pass thru, but this method is extremely labor intensive. I also had great success with corner mounting the subs in the rear of the vehicle similar to how the Stealthbox or shaker 1000 sub enclosure works.