The first thing that no one has really determined is if the hakone radio is turning the amp on.
If it's not, more wiring would need to be done behind the radio to force the amp on with the ignition.
Besides that, it shouldn't be extremely terrible.
The assumption I've otherwise made, assuming the radio is properly telling the amp to do it's thing, is that the radio is sending out high level/speaker level audio, when the amp is expecting low level audio.
To convert it, you need to cut a
line output converter into the factory wiring before the amp to make the right signal, and then a rca interconnect cable needs to be cut up and connected to the amp to give it the signal it wants.
Given your wireless subwoofer preference, just about any competent car audio shop should be able to handle it all in less than a day and about $50 ($862.41CA) in parts
But i can work up a diagram later if you would prefer to shoehorn yourself into the tiny recesses of the trunk for a fun weekend project.