Hi
Ive had there same problem , running a Pioneer xt7800 head with two door speakers and two tweeters ( pioneer component) no rear speakers.
I bought a Pioneer TSWX120A plug in woofer was going to sit under passenger seat ( and will do when I get my ass in gear and refurrb interior) but resonated to much ( needs deadening) so is just sitting on the floor between the walk through area at the mo , and really chuffed , it won't rattle your windows as such but a noticeable difference and good enough for my 40+ year old ears.
Mine needed a twin phono ( had to buy separately ) a 15amp live feed (supplied) , I took off leisure battery,a ground wire (supplied) , connect to chassis . took all of 30 minutes to install , and vast improvement .
think you need to be sure you have the sub leads/inputs/outputs on your head unit , and if you have more money/space then a separate sub and amp will
be better, but I turn mine down ( to much bass) depending on what I'm listening too.
If you have a local radio shop , sure they would demo a hideaway one , as very easy to hook up .
Chico