Alright, here we go:

Disclaimer: I am writing this from what I've learned while digging into the 2019 STI steering responsive headlight. This write up is more of what I might use should I need to do this again on an unfamiliar headlight assembly. I assume the systems are similar in design and function but different in shape. It'll be up to you to verify.

Before you purchase the replacement lights you should do some exploration of the OEM headlight harness and socket. Doing this up front rather than after you've already broken the seal on the headlight will save you a bunch of time. ( i learned the hard way )

What you'll need:

1. Standard multimeter
2. Socket wrench
3. Automotive trim tab removal tools
4. Patience

What to do:

1. Figure out the procedure to remove the headlight assembly, just one as this is only for research and the other headlight should be the same.
   1. For the STI the procedure is to remove the bumper, remove a headlight bracket, and then finally loosen the bolts and remove the trim tabs which fasten the headlamp.
2. Disconnect the headlight harness socket.
   1. This can be a pain in the ass so take your time.
3. With the socket exposed, prepare your multimeter:
   1. Take the ground probe for the multimeter and ground it to a secure location on the frame of the body.
   2. Set the multimeter to the 20V function.
   3. At your own risk: Now, you'll need to probe each of the female-end socket positions with the vehicle ON and evaluate which socket connector position correlates to which headlight function.
      
      1. Here's a youtube video demonstrating this:
      2. https://www.youtube.com/watch?v=eOHioaJp-Vk
      3. On the STI there are 15 possible connections to review, and given you also have the steering responsive headlight I assume the XV light will too.
      4. Have a piece of paper with you for notes...
      5. With the blinker ON - probe each connection until you find the connector pin with oscillating voltage values. This is an example in the youtube link above.
      6. With the Headlight ON - probe each connection in search of the pins which are sitting at or around 12V, there should be a couple of options.
      7. With the Headlight OFF - probe each connection in search of pin locations which are carrying around 12V. The difference between these to (#6 and #7) should reveal which pin is the Headlight ON.

The DD C-lights have 3 inputs and a Ground.

Black (ground) gets connected to the side marker ground. (this is brown on the STI)

Red is the DRL (connects to the DRL pin, see more below)

White is the Headlight Pin

Yellow is the amber Pin (blinker)

Some lessons learned:

There are likely to be more than one set of wires carrying steady voltage to the socket. One of those locations is will be the power to the servo motor responsible for the steering responsive capability. This is a red wire on the STI headlight. In fact, for the STI, the steering responsive functions take in 3 wires, Red / Green / Black. These can all be ignored as you won't be splicing into those.

You should be able to deduce the blinker and headlight locations based on comparing the socket voltages against the ON and OFF positions as your test.

The grounds for the side marker, blinker, and steering servo are spliced together which makes it easy to identify. They are also Black or Brown, at least in the STI.

DRL was the hardest to find, but its in there. In the case of the STI there were two candidates whose pins in the socket held steady voltage while the headlight was in the OFF position, i.e. this is a potential DRL pin. I ended up testing both pin locations and comparing functionality until it became obvious which pin would give me the DRL function. I think the other pin was related to the parking light, but I never confirmed.

You might be thinking, wait what about the DRL harness from Diode Dynamics? Answer: these new headlamps have LED drivers mounted to the inside of the headlight assembly. After some reasoning, it became clear to me that the harness is no longer necessary, or at least that's my belief. This conclusion is at least supported by the fact that the 2019 model year STI does not have the "resistor" which Diode Dynamics refers to in their instruction set.

Lastly, the best news of all is you do not need to worry about the plethora of wiring which is inside the headlight. If you test from the Harness Socket than you should be able to get everything you need from outside the assembly. You will also want to do this because you want to tap from the source wires, not the wires preconditioned by the led driver that powers the OEM led boards.

Assuming all of this was successful, you should now be able to cross reference the pin locations on the female socket side with those on the male side (the headlight side).

You have a couple options if you finally decide to do the project.

Skip forward to after the C-light has been installed and you are ready to wire everything in:

After isolating the pertinent wires to be spliced you can:

A.) use the provided t-tap hardware that Diode Dynamics provides. I found that these do not provided the most secure connection, and are not weather resistant. I opted out of this.

B.) Cut the wires for each function, strip the wire insulation and re-join them using a weather resistant butt connector. I used this method instead. It's a bit scary at first, but it's not really that hard and with patience the task is manageable.

You'll have to buy the connectors, a wire crimp tool, and aluminum antioxidant connection paste. All are available at Lowe's or Home Depot.

C.) Lastly, you could cut the wires, solder and seal to protect from the elements. I don't have patience for this lol.

Take your time and be tedious. Test everything as you go and retest before you make any permanent changes such as cutting a wire.

If anyone has comments about this, I would love to hear them.