There are two points where the ability to trigger the starter can be
interrupted.
The clutch multifunction switch which is mounted coaxially
with the clutch master cylinder pushrod.
Either the switch is physically off, the connector is off
or a wire is broken.
On an automatic trans truck there is a shorting bar that installs in that switch.
The other point where it coud be interrupted is on the wiring harness
that runs down to the reverse light switch, the engine oil temp sensor
and a couple of other things... look for two connectors on the drivers
side inner fender one is black the other grey, on one of them there is
a short wire loop...(on the transmission harness side of the connection)
This short loop replaces the wire that would normally run to the neutral
safety switch on an automatic transmission.
AD