Freightliner Columbia Question

I have a 2007 Freightliner Columbia w/ C15 ACERT that won't start. When I turn the key on I get no check engine light. I'm fearing the problem is the ECM, but before I take it out to have it checked at a Caterpillar dealer, I'd like to exhaust any other possibilities that I can. My understanding is that there is a fused wire that goes from the batteries to the ECM. When I open up my battery box i find a 12 guage wire that has a 15 amp in line fuse and a larger wire that has a 150 amp marine circuit breaker. I am not able to trace either of these wires as I lose sight of them above the transmission. Is anyone able to tell me where these wires go? Thank you.

 

Thanks for the reply. At this point, I'm not 100 percent sure of anything. I don't have a fuel pressure guage, to check the fuel, but I used a non-contact volt meter to check for voltage on the wiring harness while cranking. That indicated no voltage in the wiring harness. Also, since I have no check engine light when I turn the key switch to on so I believe I don't have power to the ECM. I'm getting ready to go back out and work on it and will probably have to take the plug off the ECM to check for certain, but have been trying to find any broken, corroded wires, bad grounds or whatever, before I do that. It looks like it's going to be a little bit of a pain to get the plug off the ECM, since it is so far back under the cab.