What you are looking for is a place to tap into the vehicle's power that can provide the necessary current with little voltage drop and little interference from the vehicle's alternator, computer, ignition system, etc.
I will tell you how I did it on my 2002 Ford Expedition which may give you some ideas. On my vehicle, there's what appears to be a 4 ga. wire running from the positive terminal of the battery to a block on the firewall just to the right of the engine. From that block, power is distributed to various primary busses in the vehicle. I added a 6 ga. wire running from that block through a gap between the inner and outer fenders on the right side. I put a "maxi-fuse" holder in that wire and armored it with split tubing. That wire then runs down behind the right kick panel where I found a rubber plug to poke my wire through. From there, the wire goes to a fused distribution block to feed various radios and other accessories. I bought this distribution block from West Marine.
The negative side of that distribution block is wired with two 3/4" flat braided wires to a large screw that goes into the floor of the vehicle.
In my experience, this installation has been trouble free except for some alternator noise that cropped up just before the alternator died. One of my radios is a Kenwood TK-790H which draws about 35 amps on transmit, so I do put my wiring to the test. As preventative maintenance, I make sure the connections are clean and tight, especially at the battery.
There are arguments both ways as to whether or not you need to fuse the negative side of your power leads. After looking at mobile radio installation guides from General Motors and Ford and listening to comments from experienced installers, I elected to not fuse the negative lead in my installation. There are also arguments both ways about running a negative lead back to the battery. Let's face it, everything in the vehicle is grounded to the chassis or body. As long as the chassis or body have solid connections to the battery from the factory, I don't think you gain anything by running a separate negative lead for the radios.