No relay involved, unless your car has been modified - perhaps with an aftermarket alarm system?
Normal wiring feed is from headlight switch through Fuse 33 (7.5 amp, interior fuse panel) then via O/BL wire to front left lamp and via O/W wire to rear left lamp. Unfortunately no means of easily isolating front from rear to allow isolation of short.
These wires originate at a 16 pin connector on the underside of fuse panel (i.e. without flipping it down) in the center at the firewall end. Wiring in trunk is usually the first place to look since damage from luggage is more likely.
BTW, not quite sure I understand your "after a given amount of time" comment re fuse blowing, since normally that would happen only when you turn on headlight switch.
Some possibility that poor connection at fuse panel is overheating and causing fuse to melt from resultant heat rather than an actual short causing an over current condition.
