I used this one and it sounds great.
CDT 6.5" Subwoofer
You still have to dremel out the "fins" on the inside of the console where the speaker's magnet is going to be, but that's not a big deal. There's a really good write up on how to do that on one of these forums. It's a dual voice coil, so it wires right up with the existing 4 speaker leads. It's more expensive than other suggestions you might get (pyramid), but it's a well made speaker and sounds great.
It worked for me.