This is how I installed my Yaesu FT-857D in my Saturn L100. I ran a 10 gauge wire directly from the battery to the trunk (fused of course). I grounded the negative side to the rear deck where I mount my equipment. Since the rear deck is part of the unibody, it is connected to the same sheet metal that the battery is connected to under the hood.
I used a West Mountain Rigrunner 4005H for power distribution in the trunk. I am using a mini Shark antenna for 20 meters in place of the stock AM/FM whip antenna. I had a custom adapter machined to convert the M6-1 thread of the standard antenna to 3/8-24 for the Shark antenna. I use a Comet CF-706 duplexer so I can use the HF radio and the FM tuner with the same antenna.
I used a MiniVNA antenna analyzer to tune the antenna. With my antenna thread adapter installed, I had to shorten the antenna approximately the same length as the adapter to get the minimum SWR in the center of the voice portion of 20 meters.
I used the hook side of some industrial velcro to attach the duplexer to the carpet above the strut tower in the trunk. The radio is attached to the rear deck with stainless steel sheet metal screws.