New to the forum and wanted to post on this thread, as it's been really helpful to help retrofit the heated leather seats! I've got the same car as the OP (2013 Mk7.5 Titanium) and had exactly the same experience.
The small piece of loom is the tricky bit as mentioned. Finding the seats on eBay isn't too hard - they come up from time to time, in either full or half leather versions - but the small connecting loom just isn't available at all.
I contacted loads of breakers and even the place I purchased the seats from, but they either didn't have it, had thrown it away or were unwilling to separate it from the shell. One breaker had lots of Fiesta ST shells and in one of the photos, I could just about see the piece of loom - but they still wouldn't separate it! I also agree with the OP that Fiesta STs are a good place to look. They get written off much more than Titanium X models.
But then I had a stroke of luck: I already purchased the heated switches, but another set came up with the centre console surround plus a cut-off loom! So I bought this and was able to solder on extra lengths of wire:
At the other end, you need to find the connector under the centre console. It's next to the gear selector. You can reach it by removing the passenger side trim below the dash and console:
Here's a close-up:
I cut this block off and used a new aftermarket connector block to join the wires together. The back of the heated seat switch has 8 wires, while the one in the centre console has 7. They all should colour match, except one, which is black/blue. This provides illumination for the switches, and it needs to go to pin 5 on the C237 grey connector under the stereo. You can splice the same black/blue wire that goes into the auto stop/start, ESC off and park assist buttons nearby if you wish (I tried this and it also illuminates).
Otherwise, all plug and play. The leather seats have more wires in the connector block but as the OP says, check your fusebox. If you already have two 15A fuses for heated seats, then you're all set.
A worthwhile upgrade! They're more comfy and supportive than the Titanium versions, plus heated of course.