Where do I tap the line level converter into the vehicle, to hook up an aftermarket amp and sub on an 04 H2 ?

I was thinking one of the rear speakers...yes, or no?
Is there a factory sub I should try and tap into? If so where is it?

I dont think this should make a diference, but just for reference,04 H2, Scotch line level converter (from wallmart),
1000 watt mono block amp, 2 12' sub enclosure.

Thanks in Advance...