I did that "in spare tire sub woofer" which others have installed and was posted on here before. I got the Bose sub for about $45 on ebay. It did not have the internal amplifier. Bose makes really good well engineered products.
I found a good bass amp. Boss R1100M, 1100 watts peak power and a class A/B amp. Class A/B provides cleaner sound rather than the class D's which are being used for sub amps. I paid $47, new for it plus shipping. It looks real quality.
I am placing the Boss Amp under the passenger seat. I connected the heavy ground lead under the Pod to a bolt used to hold the old Blau amp. No need to drill holes! The + was run through the passenger side from the rear. I fished the wire through to the side speaker hole and ran it down. No need to pull any thing interior wise.
I placed the Boss level control on bolt which holds the passenger try on. I had to use a longer bolt to accomplish this. But the control is not something you adjust often.
My head unit is a JVC I had installed awhile back. See previous posts on here.
End result......FANTASTIC. It provides just enough bass, more than enough and made a real improvement. Amazing for a 6" Bose sub woofer! The other best part, looks all stock, no boxes, stock looking.