Join Date: Jun 2015
Location: Sydney Australia
No problems, happy to help.
Might be slightly different on the High Performance stereo, but with mine there's an amp/dsp under the passenger seat. Someone on this forum kindly identified the pinouts we need to use to pull a speaker-level signal. It's the green connector here, and the wiring is as follows:
1(Grey/Black) & 9(Green/Black) - Left Front
2(Grey) & 10(Green) - Centre Front
3(Yellow/Black) & 11(Violet/Black) - Right Front
4(Grey/Red) & 12(Green/Red) - Left Rear
5(Yellow/Red) & 13(Violet/Red) - Right Rear
For a sub amp, you'll need a left and right signal so you would use #1 and #3 here.
As you've eluded to, you'll either need a mono amp with high level input, or a mono amp plus a LOC (line-out converter). The most difficult part you'll find is the wiring of this. If you're not confident, I'd have a shop do it. Should only be 2 hours' labour. I'm installing mine in the spare wheel cavity to the front/left. It will sit just above the spare tyre, so will still have access. I'm running a 4-gauge wire from the +ve battery terminal, into a fuse, then into the front passenger footwell. There's a nice large grommet there you can fit it through. From there, running it down the side of the car under the carpet and into the boot. From the amp you simply run a short ground cable to the chassis. I'm drilling a new hole to do this, will sand the chassis, then seal it top and bottom (i.e. under the car) with bitumen to ensure it's completely waterproof and won't corrode.
I've gone with a 10" sub because an 8" probably isn't significantly different to the stock system (let alone your High Performance). My enclosure pictured is 25 litres, but you can get slimline 10" subs that only require maybe 15 litres.
So you'll need sub in a box, mono sub amp (preferably Class D for this), wiring kit, and potentially a high/low converter. Let me know if you need any more help!
Oh one thing, I'm assuming that the standard head unit doesn't limit low frequencies to the main speakers (to protect them). It may for example have a high pass filter set to maybe 45Hz. If it does, then the sub isn't going to receive frequencies lower than this. If this happens (fingers crossed it doesn't) then there are only two options. Firstly use an LOC that restores bass (e.g. Audio Control LC2i), or go for an external DSP. I'll let you know when I get to this stage but am fairly confident we'll be ok. Good luck!