2003 H6 4AT (not mine) was experiencing immediate blowing of fuse #5 (10 Amps) in the cabin fuse panel when lights were turned on. Fuse #5 is in the wiring carrying power to the external clearance, licence plate and tail light sockets.
Owner had checked the 1157 bulbs at the rear (4) and front (2) for the usual faults – flattened contacts on the base of the bulbs, broken filament shorting inside the bulbs, and visible shorts at the contacts in the sockets -- everything looked good. (Bulbs were changed in any event.)
I was called after a Subaru specialist shop suggested changing the Parking Light switch on top of the steering column. That switch has been found to develop faults (there's related posts here) but I couldn't see how the switch could cause fuse #5 to blow because the fuse is after (i.e., downstream of) the switch. A short that blows the fuse has to be after the fuse, so the switch didn't seem a likely cause.
I started by measuring at the cabin fuse panel from the downstream contact of fuse #5 to ground, and sure enough there was a short.
The front 1157 bulbs (for clearance and turn signal) were removed; no change.
The hatch harness connectors were disconnected from the rear body harness in the cargo area, but that didn’t change anything, so the short wasn't in the hatch.
Next, the combination light (outer fender light assemblies) harnesses were disconnected, in turn. These are the short harnesses -- about a foot in total length -- that go to the two bulb sockets (tail/stop and turn).
With only the right side combination light harness connected the short remained, so it was in that short harness or its tail/stop light socket. When flexing the harness the Ohmmeter varied. There wasn't any apparent fault in the harness itself. However, when measuring between the socket metal housing (ground) and each of the (two) contacts inside the socket, the short appeared and disappeared when pressing down on the spring loaded movable disk in the socket.
In days past I’ve seen the wires in the socket bared by overheating and shorting to the spring that presses the contacts against the bulb base. But there was no sign of overheating.
Here's a photo of an 1157 socket similar to the one used for the 2003 rear tail/stop light:
The metal part of the socket was then pulled out of the plastic base. Now that the interior area where the wires come through was visible what had happened was apparent.
The wires come in the back of the socket and go straight up through the center of the spring to the contacts. But there appeared to be a small piece of shim-like metal caught in the spring.
The disc that the two contacts are on (some twisting and pushing), along the wires behind, were removed from the metal housing. This revealed that the thin piece of metal was a thin washer, one of two in the socket, with outer diameter to fit the housing and inner diameter slightly smaller than the opening in the base where the wires come through. The errant washer was jammed in the spring on an angle, (see above photo), half-way along its length, and was bent like a spring washer. The other washer was down at the bottom end of the housing below the spring.
With the bent washer moved toward the plastic disc, it was apparent that the sharp inner edge of the bent washer had been pressed against the tail light wire, and and had cut into the insulation and could touch the wire inside.
The bent washer was straightened, moved down to the bottom end, the cut in the wire insulation was sealed, and the socket reassembled with both washers under the spring. I haven’t yet figured out what the two washers are for or how one washer ended up in that odd position where it cut into the wire.