So I did it to 14 Recaros into a 13GT w/401A and I added power and driver lumbar too. I can speak for that process but s550 may be different.Any recommendations for adding seat heaters to 2017 GT350 seats?
To dismantle the seats you need to get a upholstery kit from Amazon ($20). Disassembly is not difficult if yr handy but you will definitely want to collect good instructions on the forums first. I recommend not messing with the hog rings as they can be difficult to reinstall. Same with the bezels for the headrest holes. I didn't need to for my 13-14 Recaros but 17 may be different.
There are two paths - upgrading to OEM using all OEM harness, heaters and buttons -or- adding an aftermarket system which uses separate harness, heaters and buttons. Aftermarket systems run less than $100 and the better ones seem well made. I prefer the OEM look and my car came with heaters stock so I did it that way.
IDK if you got a GT350 or if yr adding those seats to some Mustang. If yr car came with heaters you obviously have the harness and switches for it so the swap is all about modding the seats.
If not, you need to find out if you share the same car side harness (as distinguished from the seat harnesses) with cars that have heaters.
If you share the same harness you need to collect switches, and doner seats and install. In s197's all of the relays are in the seat harnesses so if you get donors yr all set. The seat tracks are the same for stock and Recaro so it's mostly interchangeable. That said, I strongly advise you get seats from the same exact year as yr car as Ford loves to make little changes. My donors were from a 12 and while I could use the power tracks and heaters the harness were slightly different so I hat to buy 2 new Ford units (only $100 for both). Also be advised that on s197's if you swap seats you need to keep yr stock passenger seat occupancy sensor or you will get a dash light and pas airbags may not work.
Now if your car did not come with heaters stock and your car side harness is different than those that did - I would just go with an aftermarket system as I bet swapping harnesses will present many problems and may in the end be impracticable. I've seen aftermarket installs where the on/off switches are installed on the seat by the stock switches and it looks good (undetectable).
One last thing, if you install the heaters in warm weather testing them can be difficult as the heat is hard to feel and seems to take 2 mins or so to develop. In cold weather tho you can really feel them. Not sure why. Best