Basically, instead of using the onboard amp in your head unit, use a separate amp. You will need a head unit with pre outs/rcas, amp/s and fitting kit. You run, rcas from the HU to the amp. The amp needs separate power and earth and a switch from the HU to switch the amp on. Its essential IMO to fit an inline fuse from the power to the amp.
A simple set up is a 2 chanel amp powering some 6x9s in the rear. Personally I hate 6x9s. I have non amp'd components in the front doors and an amp 12" sub woofer in the boot. I keep meaning to get around to amping the comps as they sound much much better amp'd. In my old ZR, I had amp'd fronts and a sub, nothing else and it sounded great and was fairly cheap.
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