Depending on how tightly the enclosure is sealing against the perimeter of the spare well, you might need to port it or flip it over. The way you have it set up along with the design of the golf's trunk is likely creating a bandpass so you'll have to consider that in your enclosure design (I know you already installed your enclosure, however).
Another consideration is whether you wired it out of phase, but the schematic you posted should have prevented that. It's worth checking since the speaker can be banging all around ("hitting") without creating the bass waves you're craving if it's out of phase.
Did you measure the interior cubic feet of the enclosure during design? I'm mainly curious whether you filled the hole with a speaker box or whether you built a speaker box and then made it fit into the hole, if you get my meaning.