91 Ford Probe won't start from the key.

For a couple of years, I just had to wiggle it a bit and it would start. About a month ago,
that stopped working. Battery, plugs and wires seem fine. The starter and alternator have
been replaced in the last 5 years. Sometimes there is a small clicking sound, but not
always. All electronic systems work. Does start when we push/pop the clutch.
How do I figure out if its the ignition switch or something else?