That's where I ran mine as well. along with boost gauge tube and EGT probe wires. I have found that the easiest way to do it is to take a piece of wire about .0625" dia. and 12" long. Bend a small oval shaped loop in one end and tread the DSP switch wires through it like a needle. (make sure you have plenty of slack)
Stab the non looped end of the wire through the grommet from the inside of the truck then go around and gently pull it the rest of the way through into the engine compartment.
thanks everyone i installed it tonight and went through the existing hole behind the parking brake. Test drove it tonight and i can tell you what this B%*#H is bad. Nick at Duramax Tuner has put a smile on my face. One of my tunes was an aggressive tune and i have already turned the truck sideways. The daily driver tune is pretty darn sweet compared to normal tune. :rockin