Ideally the pistol stays with the person, but I do have a tuffy underseat lockbox that fits a decent sized pistol. It’s the lockbox that still allows the seat to flip forward. For reference, it’ll fit my Sig 229, but it’s just a hair too small for my Smith and Wesson M+P
When I'm out and about with a concealed gun, a Kahr CM9 in my pocket, it stays with me. If I'm going to work or something where I cannot conceal carry, I usually have a Glock 19, and I lock it in the glove box.
I tend to prefer shoulder carry if possible. I have some MOLLE seat covers and have some holsters that fit. If it's raining I'll use a variety of sealed hard cases/bags or the center console. I've never found a "this is perfect" position yet, but I like playing around with where I put gear anyway.