I have a 2004 Trailblazer. The power for the radio is connected to the box under the rear seat behind the driver. (Pull the plastic cover off. You will not miss the big red wire.) The wiring is grounded to the chassis near there as well. I use power pole connectors for my gear. The radio itself sits underneath the rear passenger seat. I do not use the Onstar service/equipment, so I pulled it out. I have room to put the radio under there. On extended trips I put a Kenwood 480SAT and Kenwood D710 under the rear passenger seat and connect them with the power pole connectors. I set all of the radios to use APO (Auto Power Off) There might be a slight trickle drain, but nothing that I noticed. I use a Yaesu 7800 in that vehicle daily and a Diamond NR770HB antenna. Please note that antenna is meant for the Diamond K9000LRM motorized mount I have on the rack. While mine sits in the middle, could you mount it on the side? Another photo shows a Comet rack mount that is closer to the edge. What you cannot see is that it has a thumb screw to easily raise/lower an antenna. Both of those antenna mounts are
grounded with a 5/8" ring connector heat shrinked wrapped to a wire and then I connected the other end of those wires to screws inside the rack rail.
While the heads of those Kenwood radios get mounted below the dash when in use, the Yaesu 7800 is simply fixed to the drivers visor using a couple pieces of velcro. I ran the wires down the drivers side column. One wire goes back under the rear passenger seat to the radio, while the other runs to the microphone you see in the photo. That Yaesu 7800 head does not weigh the visor down, as it is rather light. If I move the visor down, that head stays attached to it.
This first mount below is connected at the back right lift gate. I haven't added a ground yet and the SWR did not like it at 1.5:1 VHF and 1.9:1 UHF. It's functional and it will hit repeaters around here all day long with 5 watts and that NR770HB antenna, but it does not work as well as either of the rack mounts that are grounded and offer a better ground plane with their locations. I use simplex often, so a higher antenna is needed. My SWR on either of those rack mounts barely nudged the meter using 50 watts on 2 meters it was 1:1. It may have moved it slightly more on UHF, but nothing worse than 1.1:1. I have even used an HT with 5 watts and pushed 70 watts out of an amplifier with either of those rack mounts with 1.1:1.
* The motorized Diamond mount is the one I use daily for VHF/UHF with the NR770HB antenna on it as I park inside a garage. That switch near the microphone is the Up/Down button for that mount.
Hopefully this provides you with some ideas/possibilities that will work for your specific needs.