No firewall access on new GM cars.
I have had problems finding firewall access in newer cars in general, but specifically GM vehicles. Unless they are police or fleet vehicles, they come very well sealed up.
What I have been doing is to wire the power cords to the battery, delay timer, etc. And then route them to the lower corner of the windshield. There is usually a gap where the firewall, fender and windshield meet.
I then pull the door gasket off a few inches and drill a hole or cut a notch near the door frame edge. Wires get routed through there and then go through the dash and on to the radios.
The rubber weatherstrip gasket hides this edge of metal, (I usually drill a hole instead of a notch and grommet this hole so the wires do not scrape bare metal) and secure the cable under the hood,fender and behind the dash with cable ties.