Change Turn Signal to Combination Parking/Turn Signal Light Mod

I want to share my recent mod to my Prius' parking and turn lights on the front of the car. I noticed that I often drive in the evening where there is plenty of light to illuminate the road and having my headlights on isn't needed for my ability to see the road, but that the amount of light from the parking lights is lacking for my car's visibility to others to the front. Therefore I wanted to turn my turn signal lights into combination parking lights that would come on on the first headlight setting, but would also still blink when needed to be turn signal lights. I also wanted to replace them with LEDs. Is this really needed? probably not, but the electrical engineer enjoyed doing the modification.

The next bit gets technical: The way the lights needed to work is that if parking light signal is off the turn signal blinks the lights as normal, but if the parking lights are on, the turn signal needs to turn the light off when it activates to be seen (opposite). To do this I needed to create an exclusive OR gate (gate that turns signal on if either of its inputs is on, but turns off if both its inputs are on see table below).

Parking Light Signal Turn Signal Lights
Off Off Off
Off On On
On Off On
On On Off

I found this site with a diagram for how to create an exclusive OR gate using two SPDT relays: relay-based XOR It may technically be possible to do the same just one SPDT relay, but I wasn't sure if the turn signal relay grounds the output when off, and I didn't like the idea of current going "backwards" through the turn signal relay, so I went for the two relay option.

Here's my drawn wiring diagram for relays. Sorry if this is hard to read; I had to cross out a few wire colors that were incorrect and I know my handwriting isn't the best! Where it says headlight + it should be "parking light +". Note: Fuse not shown which absolutely must be used to protect the Prius wiring in case a short occurs! I used a 10A fuse from the jump point for the battery + which you can see in the wiring picture below. Also note that this is just for one side. The same is duplicated for both sides of the car.
View attachment 115319

I bought 4 automotive SPDT relays (two for each side), 4 relay wiring harnesses, two new front LED bulbs, and a replacement turn signal relay cheap off ebay from China. Note that you need to replace the turn signal relay for this mod as well to prevent hyperflash (which in this case could wear out your relays quickly as well).

The first step in doing this mod was to replace the turn signal relay to one not dependent on the bulb resistance. The turn signal relay is grey and underneath the driver's dash (note - you must take off the leftmost vent cover, loosen the lower driver's side dash panel and remove a metal plate by removing 3 10mm bolts to get access to it). You can find it by turning on your emergency blinkers and feeling around until you feel the relay that "clicks."
View attachment 115315

Then it was a matter of attaching the relays to the car and wiring up. The + wires on the turn signal lights and parking lights must be cut and spliced into on both sides. The ground wires remain untouched. I used the locations shown in the pictures to attach the relays to existing bolts.

Passenger side relays and wiring:
View attachment 115316
Driver's side relays and wiring:
View attachment 115317

Here's the final results. Both LED bulbs are hooked to the headlight + in my hand drawn diagram and turn when the parking lights are on and both blink when the turn signal is used or emergency flashers. In addition, you can gently hear the relays switching from the outside of the car. I'm happy with the results!

Look with just parking lights on or turn signal.:
View attachment 115320
Look with all lights on:
View attachment 115318