There are a lot of good vehicle carry methods mentioned. But, for my tastes, the only valud ones, for myself, involve on body carry.
Any time something is carried in a vehicle, unless it is very securely mounted, which precludes fast emergency uses, it is subject to the laws of momentum. If your vehicle is hit or stops fast, there exists the possibility the weapon could fly away from you.
Even mechanical "holds" could fail under the right circumstance. On purpose accidents and barricades could cause rapid stops.
This is why my handgun, a knife with glass punch, and my cell phone stay upon my person. If I am awake, and able to use them, for defence, or egress I want them at hand.
This is just my personal preference.