I have an 09 Legacy SE. I pulled down the under dash cover, and up against the firewall left of the steering column is a little black box (among many) with a blue label with the words "Cont Module DRL". Pull down the white connector. In that tangle of wires, look for the blue wire with a black stripe. That is the one. Other people are right; it is a pain to pull out. The wire is held in place with a locking tab, and this is the pain. You'll need a very small flathead screwdriver to push down the locking tab. You will have to mess around with it to get it out, and you can try pushing the wire into the socket a little to release some pressure on the locking tab. I ended up using a pair of small needlenose to hold the wire, while I pushed the tab down with the tiny screwdriver. I have found that swearing a lot helps, too.
I am Irish, so my temper is about .02 seconds longer than an Italian's. I was this close to just cutting the wire, but it came out after a few minutes of stubbornness. Wrap the end in tape, and put your dash back together.