I have a yellow wire connected to my positive terminal on the battery what is it

I have a yellow wire (approx. 12-14 gauge) as well. I have a U-haul trailer hitch and a trailer light pack. VERY important in the 2002 to use the pack for trailer lights! Wire leads back to passenger side rear tail/turn/reverse light lens, where it provides enough amperage and triggers trailer lights from two bulb system to one bulb system. U-Haul still carries them as of a year or so ago. gARYJ