*Many* years ago I made the decision to not ever put a `Base/Moble' scanner in any of my vehicles after a rash of thefts of said type equipment in our community. (Luckily I was not one of the victims as I'd just gotten a new vehicle and hadn't gotten my scanners installed.)
At that time I started looking for some sort of solution that would still allow me to have a `traveling scanner'. What I came up with is what I use now.
First off I went looking for some sort of a carrying case that I could easily lug around that would be easy on my shoulder, wouldn't be all that obvious, and could be secured to me with something than just a shoulder strap. What I located were one nice looking leather `shoulder bag' with both the shoulder strap and a waist strap and a `MilSurp' `messenger bag / medic bag' that also had both styles of straps. Both `bags' have several external pockets and pouches, a reasonably large `main' compartment with several auxilary pockets that are each big enough to hold 1 HH scanner each, and a large pocket on the back of the case(s) that are big enough to a large spiral bound notebook. They also have some `interesting' extra pockets that are hard to find without really searching. [SWMBO's was made by a friend of ours who has a leather goods shop and was made to her specs since mine wasn't exactly the `style' she wanted.)
Next, I moved my `base/mobile' scanners permanently back into our house and picked up a pair of HH scanners that, at that time, came close to duplicating the scanners I had removed. At first I used a couple simple straps that I could hang the HH scanners by their belt clips from and then hang off of either some knobs or by catching the free end between the dash and the glovebox door. It was after SWMBO had decided that she wanted her own scanners that we discovered that some of those cup/can/mug holders that have a hook that could be hooked onto the doors between the window and the door `upholstery' could also easily hold a HH scanner quite well. (As a `plus' they hold the scanners right where they can be easily heard without having to really `crank up' the volume to be heard!)
These days, what with all the `basic equipment?' cupholders one finds in the newer vehicles, we still use the `after market' ones *along with* the built-in ones. Especially if both of us happen to have our `usual' scanners with us.
As for what to do with getting external power to the scanners we either use whatever auto adapter needed and carefully string the necessary wiring so that it isn't a hinderance in handling the vehicle or we just go with the batteries in the scanners. (One of the other nice things about our bags is that they make it very easy to keep extra batteries, in holders if the scanner(s) have holders, on hand as needed!)
Finally, if, like our bags, the bag happens to have `pen/pencil' pockets they make a great place to either store the scanner antennas if we have to remove them or carry a `spare' or two antennas for those times the `regular' antenna just isn't up to picking up what we might want to listen to. Oh, yeah... We have found that, as a way to somewhat `lighten the load', carrying a set of the EveryReady Lithium AAs for each scanner really helps. So far I haven't yet found any of the scanners we usually carry have any problems with them *and* as a nice `side effect' the Lithium batteries can last up to 20 or more hours in use. (The GRE PSR-700 is the most `miserly' with a pair of them giving me, with the `Advanced' Lithium AAs, approximately 27hrs, and with the `Ultimate' Lithium AAs, 28 to 28 3/4hrs of use while scanning 2 RRs, Boulder County SO, CSP `conventional', Med channels, Air Medivac, and the FRS freqs we use around our farm. Basically a `standard?' load of freqs I use around our place.)
Given how good the current `crop' of HH scanners are these days *I* figure that I'd rather `save' the `base/mobile' scanners for base use and use the HH scanners as both `portable/HH' and `mobile'. (While I will probably get a GRE PSR-900 it will be used as a `base' scanner even though it has that `remote face' option. I've gotten so used to using my HH scanners mobile I really don't know if I really would breakdown and do a mobile install of it even if it may be harder to `see' or easier to `hide'.) BTW, another nice `feature' of using HH scanners is that one can still have use of them in rental vehicles if/when needed.