If you just want bass you can go for something like an Infinity basslink or similar which is a active sub which has high level speaker input, Just tap into rear speaker wires and wire a live and earth. Most also have auto switch on when they sense speaker output.
iv'e just fitted the infinity basslink and there ok but don't expect ground shaking bass they don't produce it
what they do produce is enough to add a little weight to hondas original setup and there quite easy to wire in you only need to get to one pair of the wires for the rear speakers and power
a remote wire is not needed as it has auto sensing built in
i tapped power from the headlamp washer fuse (fuse 31 - 30 amp ) which is not used on my car it doesn't have headlamp washers ( es ) you will need a 30 amp fuse my fuse socket was empty having no washers
as already said earth can be taken from any of the bolts that hold the back seats in place
the easiest place to get to the speaker wires is from behind the passenger seat under the carpet where the cabling runs along the inner sill your looking for a brown and yellow twisted pair of wires within the thick harness
easy enough to work out once you pull the carpet back