As far as I can tell, there is no sensor. It's just a dumb relay box that controls the lights themselves. I suppose you could wire in some photo sensor,but I don't think it would be worth the trouble.
Traded in my Silver '06 Legacy Limited Edition on a
Silver '07 Outback 2.5i Limited Sedan OUTBACK 2.5i LIMITED SEDAN --
had it home just 24 hours before the neighbor put its first ding in it ...
Did it yesterday... real hard to get the wire out of the connector.
If I had to do it again, I'd just cut the wire without even pulling it out of the DRL module. IMHO