Check fuses, check bulbs. If these are good replace headlight switch.
I've seen two other post on here with the same problem and the same 2008 Tirbeca. I'd ask the dealer if there is a know defect.
Search Subaru Tribeca Questions
Subaru Tribeca Experts
Related Models For Sale
Used Mercedes-Benz M-Class"
325 Great Deals out of 10,752 listings starting at $1,900
Used Honda Pilot"
464 Great Deals out of 18,738 listings starting at $3,495
Used Cars For Sale
2008 Subaru Tribeca 5 Passenger For Sale
8 listings starting at $9,800