The standard GT wiring is a tragedy waiting to happen after 30 years!
Check to see if any of the insulation on the wires to the wipers etc. had gone hard and cracked - or even fallen off!
Another usual fault is bad earth connections - these are mainly light brown wires that go from the accessories to a screw on the body. Check that the connections are clean and tightly attatched. Some times it is the thread of the attaching screw which has corroded - the earth wires connect through the screw thread as there is usually body paint under the end tag.
Upgraded wiring (particularly to the headlights, as those wires rotate as the lights swing into place); good earths and a "one wire" 68 amp GM alternator and everything is brighter and works better.
the same thing happened to me. i tryed everything, changed the fuses, changed the wireing, evrything. it turned out to be the switch in the wiper motor. so i replaced the motor with one of my other gts.
the wipers uses what I call a 'stops blades down" ground. it is the light blue (yes, blue ground wire) on the foot pedal switch: it originates from the W/W switch. I think it is terminals 31 and 31b on the FPS.
What is does. When you turn off the WWs an 'open seeking' switch inside the w/w motor transmission keeps the motor ON until it is in the DOWN position. But the inertia of the rotating motor couple with the collapsing magnetic field on the motor at 'turn off' runs the motor a 1/2' too much; then the slip ring (open seeking switch) is back ON again. The ground wire applied to the magneticall still energized motor coil kills any current in the windings and since the 12V was already removed no fuses blow. Don't mean to make it complicated but you need that ground wire.
The slip ring shows up on Rev K of my wiring poster.