There are no intermediate connections downstream of the connector from the back of the Eonon to the end of the 15 ft extension harness.
The 4-wire plug that goes into the factory radio (which you have said is no longer part of the system) replaces the multi-wire plug next to it. That multi plug is now connected to a mating plug that also came on the end of the 15 ft extension harness, and that multi-plug and its harness are part of the factory wiring.
If you scroll to the bottom of this page:
https://www.eonon.com/Android-Car-GP...S/GA7166A.html you'll see instructions from Eonon on how to connect the 15 ft harness. As to the CAN bus decoder, the only place you can plug it, is at the end of the 15 ft extension, in the trunk.