If you have an older vehicle of a color that blends with the road and it's surroundings without the safety of daytime running lights, this might be one of the simplest ways to add them.

As of early August, 2017, All Electronics
www.allelectronics.com
has these one-watt LED's in a rugged housing with integral parabolic reflector rated at 300 ma. and 3 volts D.C. Four in series work quite efficiently on a 12-volt automotive system, since very little power needs to be lost in a current-limiting resistor or constant-current controlling circuit.

They're factory overruns from a rechargeable DeWalt worklight - just the "head" unit - 1-Watt LED, heat sink, parabolic reflector with clear plastic lens, and a housing that was designed to swivel vertically. The Manufacturer's Instruction Manual for the complete original unit may still available here -
http://servicenet.dewalt.com/documents/English/ Instruction%20Manual/N381967,DCL040.pdf

It was found that short stubs of thin-wall 3/4" PVC (used mostly nowadays by HVAC installers for condensate drain lines) would align 3/4" fittings with the open rear sides of the reflector housing perfectly, and with a coupling in between, a cap on each end and O- rings between PVC fittings and reflector housing one could have a fairly dust-tight, water-tight mounting/enclosure. The #10-24 all- thread rod you see in the picture runs the full length of the assembly through a 3/16" hole in the center of one 3/4" PVC cap to the same at the other end, clamping all the parts together gently but firmly.

The mounting stud you see at the rear (which of course could be swiveled to top or bottom as needed) is also a #10-24 machine screw, while the electrical connections facing forward in the photo (with extra nuts and washers for electrical wiring and retained by snug- fitting polarity-coded insulating tubing) are #6-32. It was felt that, even though quite rigid, the PVC fittings would be just soft enough to be reasonably water-and-dust tight with appropriate-sized holes drilled and machine screws tightened into them to connect inside wiring to the outside world - and the assembly itself to the vehicle frame. (It doesn't have to float - just shed the rain.) It would probably be wise to slip thin insulating tubing on the threaded rod traversing the length of the inside to prevent chafing from vibration and possible eventual short circuit with the inside wiring.

One could probably also build a four-light unit in the same manner, and/or if vahicle mounting requires, replace the center coupling with a PVC tee fitting and appropriate-length stub with mounting hardware bolted through the cap of the tee. Depending on the available mounting hole choices in the front bumper and grille area of the vehicle, you may need a couple of "fender washers" on each mounting bolt and/or even a custom fabricated bracket.

If the two-lights-on-each-side scheme in the photo is used, remember that not only the INTERNAL wiring, but also the EXTERNAL wiring needs to connect all four lamps in positive-to-negative series polarity. - See diagram below.

You want the lights on only when the engine is running, of course. If you have a really convenient opening thru the firewall, you can power these from the ignition switch through a 12- to 15-ohm 2-watt (or higher) resistor (or five 51-to-75-ohm 1/2-watt resistors in parallel). If not, the accompanying automatically-switched constant- current circuit could save you a lot of hassle. It will let you connect directly to the vehicle engine battery, sense the slightly higher charging voltage, and only turn on while the engine is running.

The LM-358 dual op-amp shown in the diagram will probably give the lowest standby current draw at less than a 1/2 milliamp, even though only one section is absolutely necessary. (That's surely WAY less than all the rest of the "memory" circuits in modern vehicles.) And the extra unity-gain buffer section does seem to improve stability slightly.

The N-channel power MOSFET in the prototype - BUZ-101 - is no longer available in the AE catalog, but even the lowest voltage, lowest current model in the TO-220 Power MOSFET list should work well. Measure current output between battery positive and the MOSFET Drain connection (center lead and tab) before connecting LED's. If necessary, change value of R1 to keep current around 200 ma. or at least below 250 ma. for greatest reliability and longest LED life. They're rated 300 ma. but even solid-state devices do wear out when operated continuously at maximum rated voltage, current, and power dissipation.

And do notice that NO PART of the LED lamp portion of the circuit EVER connects to vahicle ground!

The zener diode Z1 on the right of the diagram can be anywhere from 5 to 10 volts. The one in the prototype was a 9.1V that actually measured around 8.5V. So your R4 and R6 values will surely be different. Just divide your zener voltage by 13 (a good starting-point switching voltage) and look for a pair of resistors with the same ratio that will total around 100k. If you still need wider trip-point adjustment, use a higher value for R5 and make it a multiturn.

A convenient economical enclosure is one of the TB-1 to TB-4 boxes in the AE catalog with the circuit built on one of the matching PC-1 to PC-4 printed circuit perfboards. The aluminum cover makes an easy heat-sink - just be sure the MOSFET is mounted electrically insulated but heat conductive; or insulate and ventilate the cover with something like a scrap of ventilated vinyl soffit material. And the power MOSFET's mounting stud can be used for the negative output to the LED's if desired. Of course, almost any insulated enclosure with room for the circuit and a small heat sink will do. The transistor should dissipate less than a watt, so even a clip-on heat sink like AE's HS-325 or HS-7139 might be sufficient.

Safe Driving!