Unfortunately, in a nod to today's inattentive drivers, I felt the need to install one of those hideous third brake lights. Didn't want to do any drilling (that wasn't easily hidden or undone).
Didn't want one of those cheesy pieces being sold on eBay, either. Finally found one off a '95 Maxima that was doable.

Wiring: Had to run a separate wire from the dash harness on the switch side of the brake light switch. Down through the kick panel, frame rail, etc. Ground from the CD changer bracket. The wires go up through a hole drilled behind the rear window molding.

Light housing: The Maxima housing snaps to the Maxima package tray via clips in the housing to clip receptacles in the car's package tray. So the clips had to go (Dremel tool). The housing sat at ALMOST the correct angle for the rear window. Fortunately, it also sat a little too high, so trimming the base of the housing (Dremel tool with the sanding barrel) got the housing to the right height and angle. The housing had to sit up because the bottom of the window is an inch or so above the package tray insert level.
Paint (crappy match, but I can always change it later if it still bothers me while I'm driving) from the local hobby shop. I suppose the mismatch shouldn't bother me too much, 'cuz theoretically, the package tray insert, seat skins, and window moldings are supposed to match also (yeah right).

Base: I didn't want to drill holes in my package tray trim. The housing is attached to a piece of sheet metal (roof skin gauge, long story) covered with extra leftover headliner material (another great match, not) and some foam padding underneath for thickness. It's glued on with 3M 90 adhesive, leftover from the headliner install.

Attachment: L-brackets from the hardware store to the stubs of the housing clips. J-clips or J-screws from the hardware store on the L-brackets. Oval head screws through the sheet metal base into the J-screws. Tape over the screw heads so they don't scar the package tray insert. The sheet metal base is oversize to the rear of the car so I could jam it in between the window molding and package tray insert. That way, it's completely removable with no evidence of it having been there if I don't like it. The wires can be hidden behind the window molding.

It's not a perfect solution, but it's better than those awful eBay units. It works with, and only with, the brake lights. Turn signals and hazards work correctly without running this light.

This pic plus four more.

Interior shot from the driver's seat.

Side shot. Brake lights on. One 1156 bulb in the housing.

From the outside. The light is focused slightly downward, which isn't perfect. There's some reflection off the trunk lid. But with the typical rake the car will have, will be about right, I imagine.

Last shot. It's a crappy picture, taken blind while the rear of the car is against the back of the garage (no front suspension on it right now), so my apologies. This one is in the trunk where the wires come through the hole under the window molding. It's just forward of the trunk torsion rods. The plastic convolute tubing also carries the speaker wires.

Well, you didn't have to run a separate wire. If you have a car with integrated turn/ stop lamps, all you have to do is trot down to Radio Sh$$ and buy a couple of diodes, say, 5A rated rectifiers. Hook one diode to each turn /stop wire in the trunk (left/ right) and hook the other ends of the two diodes together, then run the y connection of the two diodes to your new stoplight.

Diodes act like check valves. They have a band on the cathode end. Connect the diodes (--->|---) Connect banded ends of two diodes together and to new lamp. Opposite ends go to each left, right, turn/ stop

I understand that concept, though I believe with the connection you have described, the third light would blink either one of the turn signals activated and no brake applied.