You have a grounded circuit, probably a pinched wire. The one light at a time plug & test works good unfortunately it is time consuming and waste fuses.
Go to an auto parts store with a good tool selection and look for a circuit tester, The Snap-On one I have cost like $30 bucks twenty years ago, but a Kragen at today's prices should be ~$20. The tester has a quick resetting circuit breaker that trips when the grounded circuit is energized and also has a meter with an arroiw that deflects when the circuit collapses due to the circuit breaker tripping off line. The meter is run along the wire path and when the arrow stops deflecting you are in the general (within ~8inches) are of the short circuit.
The dash lights are a sub-circuit of the tail lights and have a smaller value circuit protection so if there is a problem in the sub-circuit the dash light fuse blows and tail lamps will still operate.
The headlights are fused in the switch, not a disposable fuse in the fuse box.
The front parking/turn lamps are on this circuit as well as all markers.
I would guess that the tail lamp wiring harness has become pinched. Have you installed a radio, CB, fog lamps or any mods that required drilling into the body? When I worked as a mechanic the most common cause of tail lamps blowing fuses was bad radio installations where the wire to hook up the radio lamp was wried incorrectly. The second most was a drill or screw run into the harness from some accessory being added.
Also don't know the wiring harness routing but if the exhaust got hot enough to melt the insulation then.....well there's your grounded wire.
Rarely, I mean rarely, the filaments in the bulbs are touching each other and because of opposing voltage grounds to make the marker lamps work cause fuses to blow. Usually touching filaments make the dash lamps glow funny and the turn signals act up.
Hoped this helped.
Tom
and like to do things the hard way.. 
