2010 Chevy Express Cutaway 6.6 Duramax starter engages whenever it feels like it.

Transmission was just rebuilt/replaced according to the company that did it, it tested
fine. Put it in the truck ran great for 2 short trips. Now the starter will not always
engaged. There's no clunk like its trying. The relays don't activate for cranking. Gone
through all wires from transmission and starter into their respective body harnesses.
Can jump Sol. positive to Signal wire and starter does what it's supposed to do. I can
hold the key in the start position for 5  -  10 secs and the starter engages. Genesys
scanner won't communicate with the ECM but haven't had much luck with that before.
Took it to dealer they said that the starter shorted out main fuse panel and replaced it
although that turned into a load of crap after getting the old panel back. There was
absolutely nothing wrong with it. Can't trust the dealer for anything and this isn't the first
time had issues with them. If I hold the key in the start position for anywhere 5 - 15
seconds the truck will start up normally. Sometimes cycling the key repeatedly the
starter will engage for a second or it will actually start. I'm at my wits end, my boss is
beyond furious. Any help will be greatly appreciated.