Truck Repair: Truck wont start, starter solenoid, selenoid

Question
I have a 98 freightliner century with cumminsn14,truck wont start.replaced ignition switch,selenoid in firewall,took all ground wires off cleaned them,starter on truck is only 6months old,this is the 2nd starter in 1 year,batteries are good. What do you think my problem could be? The only way to start the truck now is for me to bump the selenoid on the firewall with pliers.

Answer
Well if it is not the starter then possible a wire to the starter solenoid on the firewall or wiring to the solenoid.Need another person & a test light to see.While someone is holding it in the start position see if one of the post on the solenoid is getting 12v POS when trying to start.If it is,check the other post for a ground.If not it is from key switch to solenoid or some safety/antitheft device.