I got a cut and splice kit for mine off eBay that supposedly is done inside the trunk, so no connections are exposed to the weather. I might try it this weekend, and will report back once done.
Got the M&M Electronics kit,
www.mustangtaillightsequencers.com. Hardest part was getting my aftermarket taillights out - the studs were tight on the sides of the mounting holes and I didn't want to break anything. Yes, I filed the holes a bit bigger before I replaced the taillight assemblies!
The instructions were text only, a bit confusing to a non-electrician like me. I found a set of instructions on their website with photos that made it a lot easier. All you need once you get your taillights out are needle nose pliers, a wire stripper, and an awl to poke a hole thru a rubber grommet. The splices and the electronics are all inside the trunk out of the weather. One side didn't work at first, but once I cursed at it, checked all the connections and crimps, and put it back in, it worked great. I did this on my wife's car, and now will do mine.