Some of the other Mustang books out there say that the GT/CS had "sequential taillights". like the '68 Shelby. They didn't.
As Joe pointed out, there are LED driven units available, and listed in the classic Mustang Magazines.
But--before you get into that, make sure that your wiring, sockets and lens gaskets, etc., are all in good shape (especially up in Alberta!). You may wish to test the signal to the sockets with a voltmeter/multimeter first.
Welcome to the group, and congratulations on getting a GT/CS! If you're not registered, go to the page on this site to do so. BTW--is your GT/CS from the B6 or B7 DSO (see your door tag)??
thanks--
Paul Newitt