Truck/Trunk Gun
Well for me it's a little bit different.
My patrol car (Ford Police Interceptor) has a dedicated trunk gun. It's a Remington 870 locked into a bracket that's mounted to the trunk. My AR-15 sits up front on the roof in another locked bracket. When I go off shift the guns come out of the vehicle and are locked in a safe.
For my personal vehicle (Tundra Double Cab & Kawasaki Ninja) I always carry my GLOCK 22 and if I do some long distance driving I take a long gun with me. My SBR PS90 in a 5.11 Select Carry Bag. Nondescript, compact, and packs a punch. The only firearm that has a permanent home in my truck is a Henry AR-7 that I paid $75 for. It sits disassembled in a tupperware container with a first aid kit, MREs, drinking water, toilet paper, poncho, and road flares. It's not a defensive gun... it's an oh crap... I'm stuck and need to eat gun. Look at that... fluffy tailed tree rat. Looks like I'm having dinner tonight!