The type of vehicle you have will have quite a bit to do with the type of safe you can install.
Here's what I currently use: http://www.sportsmansguide.com/net/cb/cb.aspx?a=282585 Its good for storage but the speed of access sucks (basically your probably screwed).
There's some on these pages that could work depending on your vehicle (in no particular order):
In my previous Jeep I had one of these which worked great (pics attached):
The day planner worked well with a medium frame revolver too. With a smaller planner a smaller gun should be just fine. I wish that they made one for the Grand Cherokee.