Easiest way is to go look between the master cylinder and fender on the firewall. There is a 2.5"-3" grommet there which the factory bundle passes through the firewall (bundle is only about 1" thick). It has become standard practice to run the power lead for the radio through there by just poking a hole through the grommet with an awl, screwdriver, end of a whip (not the end with a static ball), etc. Tape the end of your fish tape to the power lead and pull it through. On the driver's side there is a kick panel that can be removed covering access to the under dash/transmission hump area, the metal structure there is grounded and one can simply unbolt one of those two bolts and ground the radio there.
Oh yea, for future antenna ideas you may consider the Panorama Sharkee. It could replace the OEM Onstar/LTE/GPS/XM antenna but it's not noticeable when mounted further back either. Mine is the tri-band whip and is currently only diplexed out for VHF and UHF (I've been trying to find a triplexer so I can finish up the 800 MHz install work but I'm thinking about a used APX7500 with O3).