In the 1986 Miami FBI Shootout, two agents who were chasing the suspects had their car rammed, both lost their primary weapons. One was able to return fire with a holstered backup weapon he had, the other was unarmed. Both ended up sustaining injuries during the subsequent gunfight.
My advice? Keep it in a holster.
Remember that when you keep a weapon anywhere but on your body, you risk losing access to it in a collision while in a motor vehicle. Don't think you'll be in a collision? It can happen. I'd go with the words of Clint Smith, "Either SHOOT or DRAW, but don't do BOTH!"
Edit to add: Don't think you'll be rammed? My father recently had to draw his weapon, he was not forced to use it, thankfully, after being rammed off the road by three attackers who felt he had "wronged them" in traffic. My father is a contractor, not a police officer.
-Rob