My subaru legacy has wiring for an OEM sub that bolts under the driver's seat. Here's hoping the wire is plug into the aftermarket Domain sound-sat nav system (I doubt it somehow, I think the wire is an all-in-one going into the OEM sound system which may be different to the Domain).

 

Let's say sound wiring can be sorted,

 

Any suggestion for a cheap sub that is designed to mount under driver's seat and can fill the void in below 80-100Hz?

 

I'm not worried at all about quality, as you can't get quality balanced sound sitting in a corner of an uninsulated car to say the least.