get a click at the starter when turning key. starter good. battery good. suspicious of wire in rear of soleiniod
Flip the switch on the bottom of computer into service mode and see if it will start if it does you have a interlock problem- seat belt seat switch.. Other wise check all the connections on the starter and the battery terminals then check the second start relay on the inside of the frame on the right side behind the seat. Or turn the start switch real slow you could have a dead spot on the starter switch.
If the click is really AT the starter then it's the solenoid or the starter. Did you have the starter tested or why do you say the starter is good? You can run a jumper from the start terminal on the solenoid into the cab and either put a meter on it to see if the solenoid is "getting the signal" or not, or touch the jumper to a positive wire to see if the starter will crank or not.