Is there any access holes under the cab to run a wire from my backup camera to the inside of the cab?

 

Yes. I used one on the passenger side. Pop your head under your truck to find one...

 

Several. Pull the driver or passenger sill plate there are plugs there. They are closer to the front of the seat that the rear. I ran my reverse camera wire and my wired for my running board lights through the driver plug. There is also a plug on the firewall right between the accelerator pedal and brake pedal.