2011 Insignia Excusiv 2 channel converter speaker wires

[zzattack]

Got any specs on the sub (type/model)? If it supports high-level inputs you can grab those in the b-pillar which would be the fastest route. You could also choose to install the high-to-low converter in there, as there's plenty of space.
For a sub you really don't need stereo channels but if you want both you could tap into the signals behind the headunit which is a bit more work but functionally equivalent.