I'm going to give the Tang Band 6.5" "subwoofer" a try from Parts Express. I'll order one when I get back from work today. I think it should be a pretty good choice. I'll be sure to post pics and results.

Specs look pretty good...
Xmax 11.5mm
Fs 35hz
Qts .37

Qts could be better, but I think it'll be ok...

1999 Mystique LS V6 Alpine CDA-9807, Rear deck sub, Opt. TB, K&N RU-3530/MAF adaptor, custom heatshield, FSVT 17's, SCT 4-program chip, Eibach's/ST200's, Hella Fogs...