That's easy. Just look the far left or right side of the engine bay nearest the firewall and downwards, you will see a blank rubber grommet. This leads into the interior so pull that grommet out use a long wire to fish your way into the interior. You will need to remove a kick panel down by either the drivers or passenger side feet to see your wire poking through.

 

Try this http://www.jeepgarage.org/f156/new-way-into-engine-bay-without-going-through-the-firewall-78914.html

 

Hmmm, weird. I don't remember a duplicate thread?
Rob, thanks.