I am going to make a wager that the purpose for a holster mounted in a vehicle is for quicker/easier access and possibly comfort when driving - on the part of the OP.

I would agree to previous statements that one should never leave a gun visible in a vehicle anymore these days but not the op's intention IMHO.

I have seen a drink cup holder style, semiauto holder. And here

Here is a link with a few ideas. Ignore the "safe style" ones.