I have a full walk through of my speaker upgrade on my personal site but this instructable only covers how I converted the stock speakers into mounts. I just didn't think I should spend another $30 bucks on adapters when I can use the existing speakers that otherwise had no real value or use.
Step 1: Unsolder the Copper Leads
Step 2: Pry the Plastic Ring Off the Top of the Speaker Body
Step 3: Disembowel It and Carve It Up
Once you have that out then the next step is to take a dremel cutting wheel and just cut the supports. They don't have to be super smooth or pretty so don't worry about that part to much.
Step 4: Clean Up the Speaker Mount Body
Step 5: Mount Speaker to the Old Speaker Body and Wire It Up
In the last picture you can see the old speaker body with the new speaker in it mounted to my drivers side door. Since Scion (Toyota) decided to use rivets to hold the original speaker I ended up using bolts and thread-lock to put it back in (full details in my walk-through). As you can see though in the end we were able to reuse the original mount and don't have to mess with any type of wiring harness adapters.