Unfortunately, a large portion of society judges an individuals personal and professional worth by the vehicle they drive - arrive in. And that includes your clients. Even if your car is 25 years old, (and not a 'classic' by any standards) the bare minimum you should do is take it through a car wash, trash the fast food wrappers and coffee cups at the gas station and vacuum it out when you get a chance. You'll feel better, you'll look better arriving at the gig and you'll give the impression (or the illusion!) that you're a pro and in control of yourself. If your ride is at the ass end of embarrassing due to rust, decay or downright butt-ugliness; take a cab, hire a car service, or park a block away and walk.
Usually all it takes to pimp your ride is to clean it.
Get on that.