so i purchased 2 4" led cubes to use as back up lights on my 03. i have read people saying not to run them off just a switch and to add a relay in there. well i don't intend to use a switch. i want to use the positive wires off the lights and tap into my reverse light wire from my 7-pin trailer plug and ground the negs straight to the frame.. would i need a relay for this or no?

 

Depends - what is the combined current draw of these 'cubes'?

MGD

 

rated 18W each. 36W between the pair. so 3A of current

 

The reason for using the relay would be so you could run a separate "hot" source for the lights (be it from the battery/ trailer battery charge wire on the 7 pin, etc), so you would avoid drawing too much current on the factory wiring. If you use a relay, 1 pin of it would be connected to the hot source, one to the lights, one to ground, and the final pin would connect to the backup wire. This would function the same as a switch; when you put it in reverse and the stock backup lights come on, the relay will switch on, and will draw the power from your hot power source, which could be wired with heavier gauge wire, instead of the stock backup light wire, resulting in your LEDs coming on.

Edit: MGD beat me to it while I was typing haha!

 

Ok - no relay required. Good to go.

MGD

 

If you don't want to run the lights seperated and to only turn on when you put your truck in reverse, watch the video on f150leds.com. They show you how to wire it up.