Is there a specific reason you want to use a sub in this location?
You could do something over the rear axel like a band-pass and port it through your rear deck. I have a 97 240 and I am coming up with different ideas myself right now. I could see a small box with maybe 2 efficient 8" drivers built where when you open the trunk you just see a wall and the trunk still looks factory. 8s take little air space and they could work nicely there and give you punchy bass.
It's just a thought.