This is a simple project that only requires one photo to explain.
One thing that bothers me about getting another vehicle is where to put everything I had in the previous one. In this case, my previous truck had a cup holder on the dashboard, which worked well to hold my GPS. There was also room under the dash on the transmission hump to mount both a police/fire scanner and extension speaker.
Not so with my current truck, and as I use both the GPS and scanner almost daily, I had to think of something else.
As it turned out, there is a rectangular recessed area - I guess to put pocket change, keys, remote garage door opener, or something - on the top of the dashboard of this truck. This seemed to be the perfect place to put both units, so all I had to do was figure out how.
It was simple enough. I cut a piece of 1/4 inch plywood to fit the recess (about 7" by 9" if I recall) and covered it with some thin outdoor carpet such as is used to carpet wooden steps. I screwed the wooden disk I had used to hold the GPS in my last vehicle through the plywood, and ran two bolts next to it to hold the scanner mounting bracket.
The only thing I had left to do was make a power cord to run both units. I had a cigarette lighter plug from an old rechargeable lantern, to which I soldered the scanner's power cord. At the other end of the cord, I also soldered a short piece of cable with a plug for the GPS on it. (So you will have a cigar lighter plug on one end and a Y on the other, with a plug for both devices.)
One thing to keep in mind is that you will want the wires as short as possible to keep from cluttering up the dash. And I would strongly recommend an inline fuse; the accessory jack is already fused but at a lot higher current than the GPS and scanner combo will draw.
That's it. The plywood piece holding the electronics just sits in the tray, and I haven't had any problem with it sliding around. Of course, if your truck doesn't have a tray on top of the dashboard, or if you are into off-roading, you will need a different solution altogether.