Hey I made a post and havent gotten any help either...so i'll help you out if i can. In the video u posted the sub is just slapping up against the seats which isn't going to produce a great sound. Are you going for SPL or SQ? Take a look at this option
I know that it is louder firing into the front of the car like in the video. What's the "big 3"? Also, are you willing to give up ur backseats for ur system? And I have an SUV so I don't really know if the fiberglass in the trunk would make more rattle or less but the neon sounds sick Hope this helped. Here's what I'd do: If the sub is not smackin against ur backseats then go for it firing it into the cab but i would keep the middle seat down so you can really hear it or Face the sub up or facing back in a ported box with the port facing back on one side of the box but still keep that middle seat down and attach ur amp up to the back of the box where the seat is down like this but only w one amp: