I recommend keeping the original head unit due to the steering wheel controls and din size. You could upgrade to the stacker though, pick them up through ebay or similar. The original unit has good sound quality although it could always do with some boosting......
I added an 800watt amplifier and JBL 300watt 6x9 speakers in the rear parcel shelf. I found the easiet way was to use the speaker wires to the rear door speakers, and connect them to the amp via a hi-low signal reducer (pick them up from Strathfield Car Radios or similar).
I ran a huge live wire direct from the battery (via an in-line fuse), through the fire-wall at the point where all the other wiring loom passes through, under the carpet, direct to the amp (under the passenger seat). The switchable live can be picked up from the head unit, accessories or you can cheat like me and put a small switch inside the centre console. This way I can run the front speakers for quiet music and when the engine is not running, or crank it up via the switch. 800watts can drain the battery pretty quickly if it is turned up!!
The rear speaker holes are already pre-cut and it does not take much to knock out the panels. I also recommend purchasing the original speaker grilles for the parcel shelf to keep it looking original and clean. Running the speaker cable from the amp is easy once the rear seats are out, again feeding them under the carpet.
I am very happy with the results, although the amp has a sub out which I want to use as soon as I work out where to put the speakers.... Also, the amp has a cooling fan which I can hear if there is no music so I might be moving the amp to the boot.
Regards,
Mark.
