Since the subs are self-powered you should be able to tap into the signal wires for the one under the passenger seat. The only consideration is the resistance for each sub, which will be halved, since you will be putting the second sub in parallel with the first, which means you will need to know the original resistance (in ohms) for each sub and figured your signal strength as double (coming from the deck) for each sub. This is not a problem as long as you don't overdrive the head unit (i.e. crank the volume). I'm not sure how forgiving the stock head unit is but you will want to crank your volume slowly until you know what the tolerance of the system will be. Keep us posted.