Here is a walkthrough of how I installed heated seats into my 2005 Mustang GT using the OEM climate control module with heated seat buttons, and doing all of the wiring myself WITHOUT using any factory harnesses. The pictures were taken afterwards, so they are not the best.
First, take a look at the PDF of the wiring diagram attached at the bottom of this post. Here's an overview of the diagram: You will need a power source from the fuse box, either in the engine bay or from the passenger kick panel. This power will need to be split 4 ways, with 2 power wires going to each of 2 relays. From each relay you will have one wire going into the climate control module, and one going to your heated seat pad elements. Then, a ground wire coming from the elements to a grounding point.
STEP 1 - Locate a power source.
I chose to use the passenger kick panel fuse box. I used two (2) mini add-a-fuse's to have one for each seat. You could get by with one (1) if you wanted to. Here are my add-a-fuse's installed in the kick panel fuse box:
STEP 2 - Mount the relays.
Find a suitable place to mount your two (2) relays. I choose a metal bracket on the passenger side of the radio stack. To get to it, you'll have to remove the center console and the plastic panel beside the radio. I just used some double-sided foam tape to stick them on there. Here are mine mounted:
STEP 3 - Run power to the relays.
You will need to run power to the relays. Run a wire from add-a-fuse(s) up to wear your relays are. If you used one (1) add-a-fuse, you will need to split your power 4 ways. If you used two (2) add-a-fuses, you will need to split each wire 2 ways. Either way, you need to end up with 4 power wires where your relays are.
Attach your power wires to pins on the relays that *should* be labeled 30 and 86. If you used two add-a-fuses like I did, make sure to keep the pair of split wires together so that the pair is feeding only one relay. Using some crimp female spade connectors makes this easy to do.
STEP 4 - Connect the signal wire to the climate control module.
Notice in the above picture the two pink/gold spade connectors. These are on pin 85 of each relay. The green and pink wires coming off of these go to pins 12 and 13 of the harness plug that goes into the climate control module. One for the driver side, and one for the passenger side.
Now, it is likely that you will look at the harness plug, and there will be no wires in pins 12 and 13. You will need to go to a junkyard, find this plug from a donor car, and cut the plug leaving as much wire as possible coming from the plug. You'll then need to disassemble the plug and pull a couple of the wires out. You'll use these donor wires to make the connection between pin 85 on each relay to pins 12 and 13 of the harness plug in your car.
Below is the plug you want to pull from a donor car. It's called a "C294a" and plugs into the back of your climate control switches on the passenger side. Cut it back as far as possible so you have plenty of wire length to work with.
Next, you want to remove the plug from it's housing. Use a small screwdriver or pocket knife to lift up on the tab circled in red, and the plug should slide right out. It's hard to see in the picture, but the #1 and #13 wires will be labeled on the plug itself. On the opposite side, #14 and #26 will be labeled, but we don't care about those. We're interested in 12 and 13, which are in the red rectangle.
To remove your donor wires, stick your knife or screwdriver in the hole of the wire you want to remove and gently lift up while simultaneously pulling the wire out.
Here's what the wire will look like after it's removed. You'll need to do it again so you have two donor wires. Insert your two wires into slots 12 and 13 of the plug in your own car. Then connect them to pin 85 of the relays. The relay connected to pin 12 is the driver side relay. The relay connected to pin 13 is the passenger side relay now. You might want to label the relays at this point to keep track of which is which.
STEP 5 - Run power from the relays to the seats.
Use a female spade connector to attach a wire from pin 87 of each relay and run it to the hole in the carpet underneath each seat. Make sure you have the driver relay going to the driver side, and the passenger relay going to the passenger side.
To get to the hole, I ran the wire for each side out under the dash, down behind the kick panels, and under the door sill on each side.
STEP 6 - Connect power to the heated seat pad elements.
I bought a cheap heated seat kit off of Amazon. It came with 4 pads and its own complete wiring harness. I just used the pads and cut the connectors off the included harness. You'll need to find a way to connect the power wire you ran coming out of the carpet to your pad elements. There are multiple ways to do this. Assuming you have two pads in a seat, you'll probably want to put them in parallel. I tried series first (as they're shown in the OEM Ford wiring diagram) and I could feel any heat. The picture below shows the connections I made for one seat. Notice the red power wire splits to the two white wires providing power to each pad element.
STEP 7 - Connect your grounds.
You'll need to ground your pad elements. In the picture above, the blue and black wires of my pad elements come down to a single black wire that goes back through the hole in the carpet. There are grounding points behind the kick panels on both the driver and passenger sides.
STEP 8 - Install the pad elements in the seats.
There's plenty of other DIY's on how to remove the seats and seat covers. You'll need to take the covers off, stick the elements on, then put the covers back on and replace the seats in the car. Then connect the pads to plugs you installed, or whatever setup you came up with in Step 6. This is also a great time to give your seat covers a thorough cleaning or replace them.
STEP 9 - Try it out.
With all your wires ran, connections made, and plugs plugged in... turn the key on, push your OEM heated seat button (I'll assume you already swapped out climate control module for the one with the heated seats buttons. If not.. do it before you button everything back up), and hope for the best. Mine take about a minute to heat up. With the OEM switch, they should stay on for ~10 minutes and then automatically turn off.
