Sounds like you need to work out some sort of custom enclosure so that the subwoofer can be mounted in the cabin with you instead of in the back. This may not be easy.

When I moved my subwoofer from my hatchback to the trunk of my Honda sedan, I had a similar problem. Although in my case, I had a very loud subwoofer and a very powerful amp, so I just needed to turn the amp up until it was at an acceptable level.

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Tony Fabris