i am thinking of upgrading the sub in the 8 speaker system sub box. has this been done before ? if so what is the ideal size and power ? and also do you need to connect another amp or capacitor etc etc. thanks in advance.

Ive done mine.

Ive ripped the entire guts out and replaced it all with aftermarket. Its got a 100W RMS mono amp in there, going through a crossover with a Pheonix Gold woofer.

You can get a few 6.5" subwoofers but they are very expensive, not the sort of thing you see been sold cheaply. The problem is the mounting depth and the size of the magnet.

I would leave the electronics alone and just change the woofer. If your car has had the sub factory installed then the speakers are patched through it as it has 6 channels (4 speaker and 2 sub, one unused).

awesome thank you for the info, i am planning on a slimline vibe sub it literally has no big pretruding magnet so it should fit ok (i hope). going to get onto it at the weekend.