Many older trucks and cars never had daytime running lights from the factory, so there is no plug in the headlight harness for the DRL on a new set of headlights. The DRL gets its power from a separate lead that you connect yourself, and where you connect it decides how it behaves. Connect it to constant power and the DRL stays lit with the key off and drains the battery overnight. Connect it to switched power, a circuit that is live only when the key is on, and the DRL comes on and goes off with the vehicle, the way it should. Finding that circuit in the fuse box takes about ten minutes with a 12V test light.
Ground the test light to bare metal. With the key off, any fuse that lights is constant power: skip it. Turn the key to ACC (push-button: press once without the brake) and re-test the fuses that stayed dark. The one that lights now and goes dark the moment you switch back off is your switched source. Tap it with an add-a-fuse, never by wrapping a wire around a fuse.
What you need
- A 12V LED test light, sold as "computer-safe". An old incandescent test light draws enough current to upset airbag and engine-computer circuits; don't use one in a modern fuse box. A multimeter works too.
- An add-a-fuse (fuse tap) that matches your fuse type. Pull one fuse and compare: standard (ATO/ATC), mini, low-profile mini and micro2 all look different and are not interchangeable.
- A fuse puller (most fuse boxes have one clipped inside the cover), crimp connectors and a crimp tool, and a small fuse for the DRL circuit. The DRL draws very little current; a 5 A fuse is plenty unless your installation guide says otherwise.
Finding switched power, step by step
1. Ground the test light
Clip the test light's lead to a bolt or bracket on the body or frame, on bare metal. Paint, rust or a plastic panel underneath will give you false readings. Check the ground first by touching the tip to a fuse you know is live (the horn or hazard fuse usually is); if it doesn't light, fix the ground before going on.
2. Key off: rule out constant power
Turn the vehicle fully off, key out or in LOCK, push-button off. Use the interior (under-dash or kick-panel) fuse box if your vehicle has one; it is easier to route a wire from and has more accessory circuits. Touch the tip to both small metal test points on top of each fuse.
- Lights up: constant power (often labeled B+ or BATT). Skip it. A DRL on this circuit never turns off.
- Stays dark on both sides: a candidate. Note its position.
- Lights on one side only: the fuse is blown. Not a candidate.
3. Key to ACC: find the one that switches
Turn the key to ACC without starting the engine. With push-button start, press the button once without your foot on the brake. Re-test only the candidates from step 2.
- A candidate that now lights is switched power.
- Confirm it: switch back to off. The test light should go dark immediately. If it stays on for a while, that circuit is on a delay; pick another one.
If none of the candidates light at ACC, repeat with the ignition at ON (the position just before start; push-button: press twice without the brake). Either an ACC or an ON circuit works for the DRL. One small difference: many ACC circuits drop out for a second while the engine cranks, so a DRL on ACC blinks off during the start. That is normal.
4. Choose a fuse that's safe to share
From the switched fuses you found, use one for a non-critical accessory: the radio, a power outlet or cigarette lighter, an accessory socket. The diagram inside the fuse box cover or the owner's manual tells you what each one feeds. Never tap the airbag (SRS), engine computer (ECU/PCM), fuel pump, ABS, horn, headlight or wiper fuses.
5. Tap it with an add-a-fuse
- Pull the fuse you picked. Put it in the add-a-fuse's lower slot (the one closer to the blades), and the small DRL fuse in the upper slot.
- Crimp the DRL power lead to the add-a-fuse's wire.
- Orientation matters. With the fuse out and the key at ACC, touch the test light to the two terminals inside the empty slot: the one that lights is the supply side. Insert the add-a-fuse the way its instructions say for the supply side.
- Route the wire away from moving parts and hot surfaces and secure it with zip ties. If it passes through the firewall, use an existing grommet.
6. Ground the DRL
If your DRL has a separate ground lead, give it the same treatment as the test light: a ring terminal under a bolt on bare metal near the headlight, paint scraped off under it. A weak ground is the most common cause of a dim or flickering DRL.
7. Test it
- Key to ACC or ON: the DRL comes on.
- Key off: the DRL goes off within a second.
- Start the engine: a short blink during cranking is normal on an ACC circuit.
- Check the accessory you shared the fuse with (the radio or power outlet) still works.
Don't do these
- Don't wrap the wire around a fuse leg or push it into the slot next to a fuse. It works for a week, then the contact loosens and the DRL flickers or dies.
- Don't connect the DRL lead to a constant-power source. It stays on with the vehicle off and drains the battery.
- Don't splice into a random wire behind the dash because it "had power". You don't know what else is on it or how it is protected.
The short version
- No DRL plug means the DRL needs switched 12V: on with the key, off without it.
- Ground the test light on bare metal. Key off: anything that lights is constant power, skip it.
- Key to ACC (push-button: once, no brake): the fuse that lights now and goes dark the moment you switch off is your source. ON-position circuits work too.
- Use a non-critical accessory fuse, tap it with a matching add-a-fuse, ground the DRL on bare metal.
- Any shop can do this in a few minutes as part of the install. Stuck on a step? Ask our install team with your vehicle and order number, and add a photo of your fuse box.
Once it is wired, a flickering or dark DRL is almost always the ground or a loose connector: see LED headlight or tail light not working. Your vehicle's model-specific steps are in the RIGNOX installation guides.
