It may be the Ignition Switch.
If the Key is in the off position you should be getting no Voltage at the wire that goes to your starter solenoid.
I suppose another possible would be if those wires that go to the Terminal Block in front of the Battery and on the Pasanger Side Fender well are some how making contact.
See where the 2 wires run together inside of the same rubber loom (however this is on an 84 300D) if the wires are rubbed through the instalation and they make contact (+) voltage will go to the other wire.
