Pac makes a module, SNI - 35. tap into rear speaker wires with module. It is labeled Right + and -, Left + and -.
It has line level outputs to go to your amp.
JL 250/1 amp,
1 or 2 JL 10w3 sub(s). That amp will run both. JL stealth boxes are very nice.
2 gauge amp kit.
When you looking at good amplifiers, expect to pay a $1 a watt.