Well, some more info would *really* be helpfull here. Now granted it's late & I've had a couple (ok, maybe more than a couple) but I think we can all safely assume that the wiring is NOT for a sprinkler valve timer. What year & options ? I don't think it would surprise anyone to find an old aftermarket radio harness that was left in place after a previous owner took his awesome killer radio out & left all that wiring he paid so much to have installed. happens all the time. One end disappears up around the radio, have you traced the other end to see where it goes ? Is it in a factory looking loom or does the loom look different & possibly aftermarket. LOTS of trucks get sold & passed on and owners who rip out hardware they installed but leave the wiring behind. It's one of my personal pet peeves but hey, you buy what you buy & then curse the guy who had it before you. We all do it. If I had to guess strictly based on the info at hand I'd say the wiring went to something that someone else installed & then removed when he sold the truck but the wiring was left for you (or "someone else" to deal with) millions & millions of irresponsible owners do this each & every day. Those Freaking pinheads with the off-road lights and 10-speakers stereos are the worst. There should be some sort of National Foundation to help deal with it but your government wants you to buy new & more fuel efficent vehicles instead so it'll never happen.
Sorry for the Rant, it's fueled by having had to pull literally *miles* of unused & useless wiring out of truck after truck over the years.