There won’t be any sound signals coming thru the factory sub harness if it wasn’t originally set up with the factory sub (and amp).I maybe wrong, as Ive not dug into this personally, but its my recollection that all cars have the factory harmon/karden subwoofer leads built in, even if not factory equipped with it (same as for back up cam lead, siruis roof antenna lead, fwd collosion, etc.) the connections are on the stock harness.
You maybe able to simply locate that lead/connection on the trunk harness and pipe directly off of that.
If done right, it would be plun n play. food for thought.