Success!
I found the Japanese PDF for the large chip (no I don't read Japanese!) but it had an example circuit that I could read. Like the Arduino, there's an audio ground that remains much quieter from the general ground...
The pin outs are in a different order to the AMP chip (AMP is on the right, audio chip is along the bottom):
Here are the first-in-line surface mount devices I'm going to hook into:
Solder the wires on:
Wires go to a bunch of resistors:
Resistors go to the capacitors which are connected to the cut AMP inputs:
The whole unit:
So I'm using a voltage divider (a couple of resistors) to attenuate the output from the audio processor (car AM/FM +inputs radio chip):
Four of these:
Output--->6.8Kohm--->0.22uF--->AMP-Input
AGround--2.7Kohm--^
I'm using the blue resistors (1% accuracy) so the end result doesn't need fader adjustment.
The hiss has been reduced to about 1/3 (maybe a bit more). When I listened to the radio before, I had the volume on about 13. Now I have to bring that up to about 20 (which doesn't increase the hiss).
I'll see how this goes tomorrow while driving, it's too late for a drive right now. Sitting in the carport though, it was so much better.