I think leaving a gun in a truck is asking for trouble. That being said, there are lots of options for vehicle mounting. Soft, hard and even locking mounts.
I never thought about putting a scabbard/pouch on the back of the seat, but love it. I will be doing that mod here soon.
Guns in truck? It's hard to beat a glock. The 23 is midsized in .40. Good round and cheap enough to have fun with. You can put in the .357sig barrel. With that my 23 is now a 32(I think) but most importantly matches my sig p229 in .357sig.
The p229 is an awesome pistol, but is almost too light on the trigger. I know it but have had many friends mess up the DA to SA second shot badly. If it wasn't so dangerous it would be funnier. It is a tack driver!
If you have a shotgun, you need a shurefire light. My wife is modeling one on my 870 police magnum with Remington folder. The side saddle is shimmed out to clear it.
Gotta' love that pump shotty sound!
Oh, and 7.62, for when you want to kill the vehicle, too.