Okay, so we need to isolate where your short circuit is coming from, most likely upstream from the fuse box, to the light, and most likely a short to ground. What I would like you to do is with a new fuse in place, and your tail lights on, go to the trunk of the car where the wiring harness comes out of the tail light lens, and move it around, every six inches or so, if the fuse blows when you are pulling on a certain part, you most likely have a chaffed wire at this point, causing the short to ground. Also inspect where the wires go into body panels for cut or chaffed harness. I know this seems involved, but is the easiest way to find you "hard to diagnos short to ground".
Let me know if or what you find.
thanks
Nick.
Okay, a good way to find a short is in place of the fuse, retrieve an old style high beam, (low resistance bulb) like out of a 1970 chevelle or equivilent. created wires that go from one terminal of the bulb to the place where the fuse blows, and the other terminal to the other socket of the fuse (install in series), install the tail light bulb, and note the brightness of the lamp with the taillights off, then turn them on, the bulb will most likely get brighter due to a short, and your tail light will be dim, but now on. Wiggle your head lamp switch, and shake all parts of the harness, when the short is found, the high beam will dim, or flash, do to the on/off of the short. This is a very useful test.
ASE Certified Technician
Ford Drivetrain and Chassis Master, ASE Master Technician