How-To: Corner marker light as a turn signal

As written, this mod ONLY works with incandescent bulbs, see the very bottom of the page for the changes needed to make an LED work. It's a little more complicated and not required for stock glass bulbs like what I have.

I was in a fender bender recently where having a corner mounted turn signal indicator may have saved the day, so I decided to knock it out the day after. This modification allows the corner marker on the fender to still glow when the headlights/parking lights are turned on, but also function as turn signals regardless of headlight switch position! Headlights on will turn on the corner marker, and activating a turn signal with or without headlights on will flash the corner marker. I'm a big fan of added features without lost OEM function, and this mod does that.

Video:

Materials/tools:

-16 awg wire, 8 feet
-5 pole SPDT relay, qty.2 (one per side), the usual kind you'd have for aftermarket lights.
-Taps, crimp connectors

-Phillips screw driver
-Pry tool
-Stripper/crimp tool

Difficulty: 1 out of 5, pretty straight forward. Takes about 1-2 hours.

Step 1: get to the corner marker light wires. I prefer flexing the splash shield out of the wheel well. There are clips and a single Phillips head screw holding the top in. You just need enough room to reach the light. Once you can, a 90⁰ turn will pop the light out. Let it hang out of the car to give you enough room to access the wires.

Step 2: cut the black wire. This supplies a chassis ground to the light bulb. To make this modification work, we will use a relay to complete the connection as normal (parking lights on), send power through the ground and use the power wire as the ground (turn signal on, parking lights off) or interrupt the ground to the light (turn signal on, parking light on.) For this cut section, I used a 2- conductor speaker wire, with the black wire being from the car harness (chassis ground) and the red wire being to the light socket. Connect with crimp connectors or solder. Run the wires through the gap in the fender and into the engine bay behind the head light.

Step 3: tap the turn signal wire. Unplug the turn signal/parking light bulb and peel back the insulation. There are 3 wires: red/yellow (parking light), green/yellow (right turn signal) or green/red (left turn signal), and black (ground). Tap the green/striped wire, depending on which side you're working on. This will be the signal and the 12v supply to our relay.

Step 4: connect the relay. The ground from the car side of the harness (black wire in my photos) connects to the 86 and 87a pins. The turn signal (blue wire in my photos) connects to the 87 and 85 pins. The output that feeds the light (red wire in my photos) connects to 30.

That should do it! Test functionality of turn signals (or hazards) with the parking lights on and off, as well as just the parking lights without any turn signals. The corner marker should shine as usual with just parking lights on and flash whenever turn signals are used, regardless of parking lights. Be sure to zip tie, split loom, and tape the relay and wires so they are out of the way if any moving parts or panels.

So how is this working? What's going on?

The corner marker has 2 wires: a black wire that is connected to chassis ground, and a red/yellow wire that is grounded when the parking lights are off and is +12v when the parking lights are on. Our relay is wired to trigger when the turn signal is powered. Without turn signals powering on, the relay rests with pin 30 and pin 87a connected, letting the corner marker be grounded and illuminated as normal.

When the parking lights are off and the turn signals are on, the red/yellow wire on the corner marker is a ground. We use the relay to supply +12v to the normal ground side of the corner marker bulb to make it light up. The turn signal triggers the relay via pin 85 to connect pin 87 and 30, which basically sends +12v from the turn signal to the typically grounded side of the corner marker bulb. This is why the mod only works with incandescent bulbs, since they don't care about polarity unlike most LEDs.

When parking lights are on and turn signals are on, the relay switches between connecting the light (pin 30) to a ground (pin 87a) and the light to +12v power (pin 87). Since the light already has +12v from the red/yellow wire when parking lights are on, taking the ground away and supplying +12v to the other side of the bulb turns it off.


If you have LEDs in your corner lights, this is the wiring you have to follow, cutting the red/yellow wire instead of the black. It is more complicated, requiring you to pull from a fused constant 12v to power the light, and allowing a power from park or turn lights to turn the light on, but a power from both or neither turns it off. This will achieve the same flash patterns as seen in my video.