My police Tahoe SSV has provided me with several challenges, but has also opened my eyes as how to get things done. Be not afraid of the headliner.
If you want to snake wiring through the head liner, it's not that complicated. You will need something like a long and thin rod which you can tape wiring to. I have used a CB steel whip antenna. I have run power and coax to my NMO antenna mounts through the headliner, down the driver's pillar and behind the dash.
The trick to accessing the headliner is through the dome lights. The middle and rear dome lights can be completely removed with just a flat tipped medium screwdriver in less than 30 seconds.
The front dome light needs to have a couple of Torx screws removed but comes out easily enough. All dome light removal begins by removing the lens with the flat tipped screw driver, just like when you have to change the bulb except you just take it on step further and kind of pull out the whole unit. You'll see once you get in there, no big deal. Once you pull the light units aside you should be able to start snaking wire as needed.
It's tight but quite do-able. I have ham fists and Popeye arms and was able to make it work with little fuss. Well OK, I cussed a bit.
You will also need to remove the driver's front pillar to snake your wires down under the dash and to the fuse panel. This is also easily accessed. Just pry off the screw cap and use a small socket (7mm I think..). Easy peasy. Once you run your wiring you can figure out how you ant to attach your 12v socket. This shouldn't take more than an hour on a bad day.
**Although it is tempting to tap the power that feeds the dome lights, I would suggest running a separate fuse and wire to your unit. Everything on these vehicles either runs through the computer or a body control module or both. Play it safe.**
**One tip... when you run wires through the headliner, take some duct tape and tape down as much as you can. Otherwise loose wires up there WILL come to life from sympathetic vibration and drive you crazy.**
As always, your mileage will vary.
Cheers.