This is obviously an irreversible install because you have to cut holes in the "cargo compartment door", you can replace the part but i'm sure it's not cheap. I don't recommend doing this unless you have the right tools and are mechanical enough to get through some snags during the install. This is an alternative to paying for custom work that could cost you thousands. I opted not to do a full on step-by-step post because there are too many tedious modifications you have to make along the way. you just have to improvise. Here are some pics to help you get started.
Choose a desired speaker placement *If i could do it again i would position the speakers an inch closer to the front of the car from this view*. this is because inside the cargo compartment there's a lip that comes into play the further back you go. That will save you time and energy vs having to shave the lip down to fit.
This Rotozip cutting tool or a similar tool is a must for cutting this hard plastic. there are several adjustments i had to make ie- shaving down the ridges that are on the outside of the door in order to make the speakers flush with the door, and this tool helps when doing so. (cutting is messy so be sure to tape some plastic or tarp over the interior/engine vent to cut down on the mess.)
RotoZip RZ05-1100 Cutting Tool Kit
After you cut the hole, lay the speaker shell in place and draw an outline over the ridges so you can shave them until level. *the plastic will get very thin so be careful not to go all the way through*
After some shaving and some more shaving to fit you can fasten the speakers in. I recommend insulating the outline of the compartment so that there's no vibrating within the compartment.
Here a couple of pics to show how i ran the wiring
Drilled hole leading into the compartment.
I ran the wire with the support cable for the gate, i had no problem with this but just make sure it closes properly.
Run the wire under the carpeted padding on the floors, up to the dash and splice into wiring harness. This is much easier to figure out if you have an after market head unit because the factory units aren't wired for rear speakers which also means they don't let you fade front-to-back.
Once you have finished turn on your music and enjoy. There really is a huge difference in sound and your wallet will thank you for not having to spend a fortune to have rear speakers installed in your smart.
For those of you that choose to do this, feel free to ask questions if you have any hang-ups.
Brian