Your rear turnsignals are okay They are setup the same as in my '97 Cobra: the outboard two are parking lights(low bright) and brake/turnsignal/emergency flasher lights (high bright) by employing dual-element bulbs, while the inboard position's upper section is used as a parking light (low bright) only and the bottom section is your backup light. The outboard two bulbs have three wires to them (ground, low bright and high bright) while the inboard bulbs each have only two wires.
I designed and installed my own sequential rear turnsignals. I knew I had to change things about the inboard parking light to make it into the first on in the sequence, and was surprised to find that it actually had a dual-element (low bright and high bright) bulb in it. I carefully examined the socket itself and found that with some care I could drill a small hole in the rear of the socket and screw in a self-tapping screw such that it contacted the proper connection within the socket that when powered by a wire added as part of my sequential turnsignal circuit design, the high bright element of the bulb would light. I bought all of the necessary electrical parts (perf board, relays and capacitors) at Radio Shack for about thirty dollars.
The sequential turnsignal setup I describe is my own development figured out by trial and error on my workbench. I too have seen such a mod on MustangWorld where the guy uses something he calls dynamite sticks, or something. In his pictures they look like M80's. I've never seen or heard of them before, but then I'm not really into it like some may be. He did not say what it cost him. The prewired kits I've seen in catalogs, like California Mustang, are getting $280.00 with a $45.00 core charge! Of course, their setup is just a plugin job requiring only the ability to be able to figure out how to remove the tailight assemblies and unplug the wires.