For anyone still interested in adding a left speaker...
I just added a second speaker about 2 weeks ago. The Spike board is already putting out the stereo feed for the left speaker so its just a matter of tapping in.
--Wiring
The wiring is pretty simple, there is labeling at the speaker connection, pretty small under the molex connection, on the Spike board (Follow the right speaker wire back to the Spike board). This identifies the positive and negative lines for the left speaker. I used push on pins that fit the Spike connections and then used .187 terminals on the other end for my speaker. Be sure to match polarity (positive and negative).
--Mounting
This was trickier than the wiring, I ended up using a piece of plexiglass as a makeshift mount. I cut the hole for the speaker and tried to match the spacing as best as possible to the right speaker. I drilled holes through the plexi to mount the speaker directly to it and used simple machine screws and bolts. Then I drilled additional holes matching the bolts that were already mounted in the grille. I also cut up some black screen material to slip between the speaker and the grille, just to help eliminate light bleeding through. **** I was thinking about using sheet metal for the mount since that's easier for me to custom make, but I opted against, JIC it caused vibrations or rattling.
--Speaker
I matched the ohms to what the factory right speaker was, which is 8ohms. You can switch the settings to be either 8 or 4 ohm, so either will work, But I would not mix and match.
IDK, hope this helps anyone who was still toying with the idea.