Starter problem: this is a very simple circuit to troubleshoot. The interlock with the alarm system had not yet been introduced.

I suggest locating the X26/12 connector below the radio and separating it for testing with a basic multimeter. I presently have the radio and wooden console removed from a car and can email photos to you if you contact me.

Socket #5 on the female connector should have battery voltage when you turn the key to start. Next measure resistance between 5 and 6 on the male side of the connector. You should measure a short circuit. Failure of the first test suggests a bad ignition switch, while failure of the second suggests a faulty neutral-safety switch.

Convertible top: you may have a leak in one or both hydraulic cylinders which move the compartment cover (rear deck). If you remove the panel at the back of the trunk you will have a clear view of each cylinder and can check for leaks.