Seeing as the OEM winter package is $495, I decided to skip that option and install $80 heated seats after the fact.
Pads can be found in droves on eBay. Mine had a 5 position rotary dial and contained dual pads for each seat for toasty buns and a relaxed back.
For this install, you'll need the following tools:
T50 star drive socket
25' 18 gauge or heavier wire (depending on what your pads use)
2 wire nuts
Drill and bit for round switches, Dremel and cutoff disk if you have rectangular switches
Step 1: Remove the seats
On the front of each seat, use the 9/32" socket to loosen then remove the wire connector. This is the connector for the airbag sensor, airbag, etc. Use care with this part and don't bend any pins.
Next, use the T50 socket to remove the bolts that hold the seat to the car.
Each has 4 bolts.
Carefully remove the seats. Take your time and make sure the brackets don't scratch up the dash. Make sure the steering wheel is fully telescoped in for more clearance.
Bring them inside where it is nice and warm and near the fridge with frosty beverages.
Step 2 - Install the seat pads
This is pretty straight forward. Turn the seat upside down, and recline it back fully. Undo the two clips attached to the elastic Vs, then the J-clips holding the back of the bottom seat fabric on. This will be a bit tight, but you should be able to slide your pad into the space between the fabric and pad. On my sport seats, I was able to easily slide the pad all the way up to the grey accent.
Reattach the J-clips.
Remove the J-clips for the seat back's fabric, and pull it gently through the gap between the two seats. Turn the seat back over and slide your back pad into place. Replace the fabric and J-Clips.
Step 3 - Mount the switches
The dash of the focus is full of curves and creases, not much space for mounting additional switches. Plus, we'd probably have to take the dash apart to do it, and that's tricky at best and could leave to squeaks and rattles at worse.
Instead, I chose to mount them on the plastic valance near the seat height adjuster on the seat. These are pretty big, full of empty space, and a pretty good place as your hand can fall naturally to the top where we'll mount the switch.
Drill a hole for round switches, or cut the opening with the Dremel for rectangular switches. Mount the switch.
Tuck and secure any loose wires, make sure you leave enough slack for the seat to move on its track.
Step 4 - Route Power
Back in the car, it's time to run power. In my SE hatchback, there are two 15A fuses in the rear fuse panel. These were installed for OEM heated seats, even if they weren't installed.
With you spool of wire, cut two lengths. Start at the fuse box, run it along the door sill trim, and into the driver's side seat area. Do the same for the passenger side, cutting across the driver's side area, through the gap on the bottom of the center console.
Strip the end of each wire. Pull the 15A fuses, shove one end of the wires into one of the slots where the blades from the fuse would go. Then reinsert the fuse, holding it in place.
Tuck the wires under the sill plates.
Step 5 - Reinstall seats
Before you reinstall the seats, use the Dremel with a sanding drum to clean the paint from around one of the bolts on each seat. We'll use them as a ground.
Make a loop in the ground wire, and secure it between the frame and seat bracket.
Finish reinstalling the seats with the bolts.
Step 6 - Connect wires
Connect the positive wires from the seat pad to the power leads you threaded from the rear fuse box.
at this point, test to make sure everything works. Possible problems could be that the ground wire is not making good contact with the patch you cleared around the seat mounting bolt, or a positive wire may have come loose in the back.
If everything works, use the wire nuts to secure your connections (or solder them).
Tuck any wires away and zip tie securely. Enjoy your warm seats!