I haven't done it myself yet Krabby but my understanding is that you need to connect through the high level input (speaker wires) to get the signal that needs amplifying. I am 99% sure theres no rca outputs like we used to have, back when a head unit wasn't integrated and could be swapped with two hacksaw blades in under a minute.
These speaker wires are in the control module which in a right hand drive is under the steering wheel behind the cowling. I'm not going to lie, is a bugger to get to and probably needs to be loosened off to reach the speaker wires.
The easiest way is to insert a line output controller (LOC) between the speaker wires and the amp, this essentially converts the speaker wires to rca cables (with some benefits).
I'm not there yet myself but planned to use the Audio control LC1 or 2 For this purpose. One of them (not sure which, maybe both!) also uses the speaker wires to allow remote turn on of the amp. Essentially, if the speaker wires have current, it will switch on the amp. Whilst you'll still need a permanent live and solid earth for your amp, it saves buggering about finding an ignition live.
I see the LOC as a solution instead of the expensive amp option others with more disposable income go for. Also, I hanker after an old Rockford Fosgate amp from back in the day...
I hope that helps, here's a link to the LOC
https://audiocontrol.com/car-audio/factory-system-upgrade/lc2i/
I hope not to have steered you wrong here, I am sure someone else can add more detail or correct where needed.
