Head Light Relay

   This is easy to do. You will need 2 SPDT relays, some 12-14ga wire, some 16ga wire, 2 fuse holders 20A or more or 1fuse holder 40A or better water proof, and some misc. connectors. Using this method you wont have to run any wires inside the vehicle its all under the hood. Also you could use 1 very high current relay for booth lights but because some vehicles have differant left and right headlight circuits its not recomended also the one relay would cost way too much.

   Make these connections behind each headlight:

   To start out find your highbeam and lowbeam wires first then cut the low beam wire. Using some wire the same size not smaller connect contact #30 on the relay to the side that goes to the bulb. Using same size wire connect contact  #87A to connect to the other side of the cut lowbeam wire. If you use stock wattage lights use the same size wire you just used on the lights and connect contact #87 to your positive battery terminal with some type of fuse or circuit breaker in place. (if you have high wattage bulbs use bigger wire). For the next 2 connections you can use any size wire you want as thay are low current. Contact #85 goes to ground. Last of all contact #86 gets spliced into your highbeam wire. Repeat on other side and you are set.

   I recomend useing water proof connectors and covering the bottom side of the relays with hot glue or silicone to prevent moisture out the other sides should be sealed.

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The solution that genehewett posted looks like it should work but it looks like a little over kill. Usually there are two relays existing in the car/truck for high beam and low beam headlights under the hood. The easiest way to make them both turn on at the same time is to find these two relays and find the terminal on the relay that activates the relay. (IE: The terminal that switches to ground or 12V when you turn on your healight switch, you can test for that with the relay unplugged). After you find the terminal that activates the low beam relay do the same for the high beam relay. (IE: The terminal that switches to ground or 12V when you turn on your highbeam switch, you can test for that with the relay unplugged) If both of these terminals have the same polarity (IE: both go to ground or both go to 12v when the switch is turned on) then you can simply use a 14-16 ga wire and jump the two together. This way when you turn on your low beams your high beam relay will turn on at the same time. With this set up you will have no choice but to have both highs and lows on at the same time anytime you turn your headlights on. If you want to have the option to set them back to normal you can simply put a toggle switch in the jumper wire. You can leave that in the fusebox under the hood or run the wires into your cab, its up to you.

Hope this helps