First, I want to thank MZ6ZoomZoom (Mazda6Club) for originally providing this info, I did this mod using his instructions. I made revisions on some of the steps which I think make the process easier (at least for me) and neat. I also took pics of the process. Lastly, thanks to biofreak (Mazda6Club) for pointing me to the instructions. Before the mod, only the two outer lights (and spoiler) would light up when applying the brakes. After mod, both inner and outer lights light up when applying the brakes. Hope this is helpful to future people doing this mod.
Here's the How-To:
Estimated Time To Do Mod: 45-60mins
Tools and Equipment Needed:
*Sranded Wire (I used 16/2 Low Energy/Low Voltage Lighting Wire; bought 12 feet and had like 5 feet left over) FYI: MZ6ZoomZoom recommended 12 gauge speaker wire, I used 16 gauge outdoor lighting wire and it works fine.
*Multi-purpose ties (optional)
Step 1) In order to remove the fasterners use the flathead screwdriver to pop up the center locking mechanism then pull fastener out. Remove all plastic fasteners from top of trunk in order to remove the carpet. Then, remove just enough fasteners from the left inner side of the trunk wall in order to move carpet and expose rear of left brake light and wiring.
Step 2) Remove cargo net holders (left side only) by uncrewing them counter clockwise. Removing the cargo net holders makes it easier to move the carpet out of the way to expose the break wiring.
Step 3) Pull the sealers off the top and bottom to expose openings. Insert one end of wire through the top of trunk and bring it out the top sealer (left pic). Though the pic shows the finished result, right now you just want the wire coming out of that hole.
Step 4) Grab the OTHER END of the wire and insert it going up in the rubber tubing until it comes out. The rubber tubing has some electrical tape that you're going to need to remove first.
Step 5) Hope this isn't confusing. So now you have one end of the wire coming out of the trunk hole and the other coming out of the sealer (steps 3 and 4; shown in black in the pic below). Now, bring out as much of the wire as you can (shown in red in the pic) through the sealer (without letting the wire coming out of the trunk to go in). Then, tape (blue) both ends of the wire together. Once taped, pull the side of the wire coming out of the trunk back in; causing the wire end coming out the sealer to go in the hole and up the trunk. You're basically "fishing" the wire up the trunk now. I found it easier the insert the wire through the bottom of the rubber tubing rather than the top. If you want to try top to bottom instead go ahead, the result will be the same.
Step 6) Once you've "fished" the wire up the trunk, pull out just enough wire to comfortably reach the rear of the right break light. The easiest way to do this is to run the wire alongside the wiring already installed. You can use electric tape to TEMPORARELY secure the wire in place.
Step 7) Now back to the sealer/rubber tubing. Dismount the plastic casing holding the wiring in place on the left hinge. On the bottom of the plastic casing is a flap you're going to need to open; you'll be running your wire through here. Opening the flap was a pain in the ass. Once open, run your wire alongside the intalled wires, using some electrical tape if desired in order to hold both wires together. Now insert the wire through the top of the bottom sealer. This was also a pain in the ass. The inside of this sealer is filled with some kind of glue/goo which made getting the wire through difficult. Run all the wire slack through the frame opening.
You should have more than enough cable to reach the rear of the left outer brake light. You only need enough wire to comfortably reach the rear of the outer break light; you can cut the excess or bunble it up (I cut the excess).
Step 8) Back to the top of the trunk lid (the other end of the wire). Now you're going to need to split the wire in two, one half running to the left inner break light and the other to the right inner break light.
Step 9) Now strip about 1 inch of each half of the wire, twist the stranded wire. Bend the tip of the wire in the form of a candy cane, this allows 1) better contact when inserted and 2) holds the wire in place better. If at the end of the steps you try your break lights and the inner ones don't turn on, most likely this is the step you'll need to check. The problem is that the wire ins't making good contact. Try making the "candy cane" bend bigger and making the stripped tips wavy.
Step 10) Insert each bent wire end in the third EMPTY hole of its respective break light. Insert the wire as far in as it will go; you shouldn't have to force it in. Don't secure in place with electrical tape yet, you want to first check that the complete setup works. Wire should be secure enough on its own in the meantime, electrical tape will be used after checking it works properly.
Step 11) Back to the rear of the outer left break light. Your're going to cut away an inch or two of the electrical tape surrounding the wires that lead to the left brake light. Don't cut too close to where they plug into the break light, instead do it just after the big bundle of wires splits into the break light wires and turn signal wires.
Step 12) Once you've removed the electrical tape, 5 wires are going to be exposed. Find the green and yellow wire. (MZ6ZoomZoom's wire is blue and yellow so basically find the wire with the yellow stripe.) This wire is the brake light power source. Strip a small amount of wire away. I just did a 22 gauge cut with wire strippers, then just push it down some. You don't have to actually remove any of the green yellow wrapping, but if you want to go ahead.
Step 13) Strip both sides of the remaining end of your wire. Twist them together and wrap them around the section of green yellow wire you exposed. Now before securing and electrical taping everything, check to make sure it works first.
Step 14) Electrical tape everything up. You can also use multi purpose ties to secure the wire going across the top of the trunk to the right inner break light.
Check out the neat result:
Here's a link to MZ6ZoomZoom's post