I ran a strip of aluminum diamond plate (about 1/8 inch thick, IIRC) between the sun visor retainer clips as a radio mounting point and mounted my radio to that plate. I tap-spliced into the hot lead for the vanity mirrors to power it (through the radio's own inline fuse holder, of course), and ran a ground direct to the body using a self-drilling screw into the structural rail right above the windshield. This gives me a constant-on 12V feed to the CB, so I don't need the ignition in ON or ACC to use it (if I'll not be running the truck for a while, I have a quick-disconnect right behind the radio so I can unplug it completely to remove the slight draw that allows it to remember the selected channel when switched off)
The antenna line is routed above the headliner to the back, where it dips down and transitions to the outside through the same body holes where the tail light harness comes out (this is a '92). From there, it runs up along the liftgate gasket to the roof.
The antenna itself is a simple stainless whip similar to the "President Iowa", mounted to the rear roof rack crossbar using one of those mirror mount brackets. I've got a 10-gauge wire running from that crossbar to the rack bracket, which is giving me good ground to the body.
Tuning of the antenna to the radio once installed is crucial to good results - I'm seeing VSWR of right around 1.6 at both channels 1 and 40 with the setup as I have it now; while I've never really tested my range, as far as I can tell it's working pretty well for within the group on a trail ride.
Also, so I don't have the microphone cord swinging around by my rear-view mirror, I have a home-made extension cable for it that runs across from the radio to the driver's side A-pillar, down that to the bottom of the dash, and across behind the dash to the driver's side of the center console, where the end is secured using one of those foam adhesive-backed zip-tie squares. The microphone's attached to that, and lays nicely out of the way on the notch between the driver's seat and the console. Easy to reach, and doesn't slide around when I'm off-road.
Having the radio up high makes its built-in speaker work better (IMHO), but this Cobra does have jacks on the rear for PA and an external speaker, which I suppose could be routed to the aux-in on your regular radio if you needed more audio power.