They make boxes that fit under the rear seat. I have one in my '01 QC. I got mine from Ebay for under 75 bucks and it was well constructed and sounds good to boot. Go to Ebay and search "dodge ram sub box" and take a look
because the o5 has 2 metal cubby holes under the seat, with only about 4" of clearance between the top of the cubby holes and the bottom of the seat. The enclosures for the 05 would fit down inside the cubby holes.
They changed the style under the seat of the new body style. The only way to fit speakers under there now is to have them facing up into the seat. The best way to do it is either buy one of the expensive pre formed plastic boxes, or if you're willing to put in the work, fibre glass your own box. I'm hoping to start my project in the next couple weeks. I'm going to glass the bottom, up around a 5/8" MDF frame, with another 5/8" MDF top. It should sound good.
You can buy one of the cheap boxes off E-Bay, but I don't like them too much. Most of them are too small of MDF.
Another company out of Texas, I believe they were called ProBox, makes a box that ports out the back on the box. The theory was that the bass has to travel up around the cab, off the windshield, and into your ears. He made it sound like an amazing product. I would have bought one if I were a lot closer. These boxes also come with their own speaker (that look impressive). I would love to hear this setup myself.
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