This sub can get it's power either way. It has both speaker level inputs as well as line level (RCA). Most head units have RCA line outs that are for amps, which is the same in this case as what you are actually doing is powering an amp. Dimensions are 11.8in x 8.3in x 3.2in, small enough to go under a seat and be invisible. If you don't have the extra outputs, maybe that's what that adaptor is for.
I had a similar unit in my 2002; sure does fill out the sound- without sacrificing the originality of the rear deck,door panels, etc.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick