A while back I seen a cop with spot lights on the back of his truck, when he put it in reverse they came on. I thought that was cool so I got some, but they are just bolted in. Right now with work slowing down I have some time to do the stuff I meant to do a while back.

Anyway, which wire to I splice into in the cluster of wires that go to the light?

 

first how much power will they draw? if it was me i would wire a dedicated 12v source from the battery to a relay and then a trigger wire from the backup lights to the relay and the aux lights on a relay

 

I have no idea how much power they draw, they are the ones you see on a lot of the semi trucks. I found one in a scrap yard and took the lights off of it.

 

i would wire them like i said unless you want a rolling BBQ pit

 

Take the tail light cap off the truck and pull out the cluster. Find the reverse bulb. See which wire powers that bulb. Splice into that wire.

 

Under drivers side kick panel...
Black wire/pink stripe...
Check if that's the correct wire in YOUR truck, that's what I used to trigger the relay for my aux reverse lights... comes on when in reverse

 

Mine is the same pink striped black wire, but it is in the pass. side kick panel.

 

Well, I guess I'll check both sides haha. Thank you.

 

Don't know if u can do this with those but my under tailgate led bar is spliced straight in to the wire going to the reverse bulb in my tailight

 

LEDs don't draw enough power to create a problem which is why this is ok. If you draw too much power from the wire like with the lights the OP wants to, you'll burn your truck to the ground from an electrical fire. You never want to pull that much current from a wire like that.

To OP:
Just wire in a relay, better safe than sorry. You probably think your truck is hot now, just wait until that wire overheats, better have the marshmallows and hotdogs ready.