No, I did nothing other than plug the new head unit straight in....
In trying to unravel how the system is configured with Nodge68, it became apparent that the head unit itself is providing an amplified main speaker output. Whether the main speakers are further amplified by the amp under the seat is not certain. What is certain is that it must tap the main speaker output and provide the amplification for the sub, because there is no provision at all at the head unit for this to happen directly.
Attached is my sketch of decoding the pinout on the plug into the amp under the seat.
If you can see through my scrawl;
Power is on 5
Gnd is on 26
View attachment 214716
You might find that even if that amp doesn't provide any further amplification to the main speakers, it may still need to be powered to allow it to pass through the device......
I didn't investigate the source of the power on pin 5, once I'd established that it was present.... Hope that helps....