Use the factory wiring from the headlight switch to power a relay, the best way is to use relays and good wire, GXL rated insulation. Crimp and solder the connections. Output + from the relays power the lights. Upgrade the ground wires as well from the core support to the headlight ground, and from the body ground to the battery.
To wire a relay, it's really pretty simple. You have power in ( fused, from the battery feed ).... most trucks the best place to get this is one of the 2 studs on the back of the under hood power center. This gets connected to terminal #30 on all the relays. Next, you have to hook the wires coming from the headlight switch to the relays. This can go to either # 85 or # 86, however the opposite terminal gets connected to ground. #87 is the output that would go to the lights. If you are using a 5 pin relay ( most likely ) do not use terminal #87a, that would be the normally closed terminal. ( when there is no power applied to the coil )
To minimize voltage drop at the headlight plug ( assuming 9005 / 9006 ) use at least # 14 wire. You would want to find aftermarket pigtails with larger wiring, since the factory wire is smaller.
Something like this:
http://www.ebay.ca/itm/Connector-Plug-Pigtail-7-14-ga-for-9005-and-9005XS-bulbs-P20d-base-/400666631122?pt=Motors_Car_Truck_Parts_Accessories&hash=item5d49979bd2&vxp=mtr
As always, make sure all the wiring is installed in looming, routed correctly, not pinched and clipped up so it can't wear, chafe, etc.
If you have daytime running lights, you have to either pull the fuse for DRL and manually turn on the headlights, or just don't put in a relay for the low beam portion... as the coil requires 12V to close the contacts ( #30 to #87 )
It's not a trivial job to toss in a bit of wiring, it would take you the better part of a half a day to do it correctly, pulling the stock wiring out, locating the relays ( I have 2 under the computer ) drilling holes, soldering, heat shrink... etc.
