There is one wire which, if powered, energizes the solenoid and starts the starter motor, and that wire is getting powered at the wrong time.

Problems in the switch part (not the lock part) of the ignition switch can cause this, and that is what a lot of the posted answers are addressing. E.G. clean the switch, lighten the key chain, etc.

Your 1986 was made with the main wiring harness that has the crumbling insulation problem. In some models, 1983 to 1987 are afflicted, the harness has been repaired or replaced. The harness contains that aforementioned one wire.

The wire, yellow (not certain), goes from the starter into the harness and thence to the large gray connected mounted near the center of the firewall. That connector is the terminus of many of the crumbling-insulation wires.

Yes, you can replace the ignition switch starting connection with a push button, by by-passing that wire. Problems are in retaining key-to-start requirements, and just plain wiring access problems.

A word of caution: First, disconnect the battery negative terminal before getting into any wiring. Second, that yellow wire connects to one of two spade lug connectors on the solenoid. Make a note of which one the wire is on, and connecting it to the "wrong" results in nothing happening so you must rach down in there and put the connector on the correct lug. A small PITA.

Good Luck,

Bob

:>)