Welcome to HEF darrellwatkins. :drinkup
Raise the cab, locate the harness where it comes out of the rear of cab and goes down to main frame. Where cab harness meets main frame harness you'll find a rectangular connector. I think it's a red or red/white wire you need to check, it's the wire that sends power to everything at the cab control panel, including key switch. Pull that connector apart and check the pin/socket for that wire.
I had a T300 that had a problem with that wire connection once. Turn the key on, control panel lit up. As soon as you hit "start", everything went dead. I ended up probing that wire on the cab side harness of the connector with a test light. Turned on ignition, test light lit up. Hit start, test light went dead. Moved test light to main frame side of connector on that wire, it stayed hot with ignition on as well as when start selected, pretty much confirmed bad connection of pin/socket inside connector. It was on a job site so I simply spliced in a jump wire to bypass connector.
