[P156KWJ]

I've got a few other things that's making me think I've got alternator problems although they are currently filed under 'ignore and it'll go away'... hopefully

I've got aftermarket adapters in the back of the stereo to accommodate a modulator to a standard DIN setup so that could be a cause if they are of low quality.

If I was to obtain a powered subwoofer with an inbuilt amplifier would tapping in to the existing speaker wiring at the back of the head unit be adequate regardless of what the factory setup is in terms of whether it has an amp or not? Not looking to vibrate the neighbourhood windows to death but looking for a more thorough music listening experience without spending daft money