as long as you're not using a Wagon Gear tailgate, you might consider mounting a 10" sub in the tailgate:
I used 2 nails and a string to mark the 9.25" opening that is required (some speakers might need slight larger). I used a cutoff wheel on a dremel tool, then ground off the burs and vacuumed out the debris. I then painted the exposed metal so it didn't start to rust out...
using the mounting clips as reference points, cut out the same diameter hole in your tailgate liner (a rotozip or other rotary saw is really nice for this).
Measure and cut a piece of 1" MDF to fit the area to eliminate vibration in the tailgate and to give you more mounting depth. Wrap with carpeting or other material.
While everything is out of the way, go ahead and run your wire through the rubber tube used for the door lock control wire. Also, it's a good idea to put some sound dampening in the tailgate while you've got it open.
Lay everything in place (liner, MDF, speaker) and ensure it all lines up.
If you're good, then go ahead and drill your holes for the speaker mount, then remove the speaker. Depending on the screws provided, they may or may not need to go through the tailgate itself. Check your depth before you drill to prevent extra screw holes in the sheet metal (I HATE rust, so this was important to me....)
You'll need to secure the MDF to the liner and the tailgate. (I used 6* 1 5/8" deck screws) Pre-drill the holes, then screw em in. I split the difference between each of the speaker mounted holes.
Connect your speaker wire, install the speaker, enjoy your upgraded tunes!
Edit: Adding some pics showing measurements of cuts, etc.