If I was you I could have put my foot on the seatbelt of either the driver or the passenger, whoever you were behind. The seatbelt sticks out a bit at the bottom of their chair and you can restrain someone pretty well with that. Then you should have controlled your gun better. You had the situation under control but somehow he got ahold of the gun? That is weak.
Sounds to me like you were all scared to actually shoot someone. Like he just reached out and grabbed the gun from you and you didn't know what to do. Same as you did to the girl. To be honest, in the future I think you should skip the gun. You don't exactly sound like a marksman but you do have the experience in hand to hand. Skip the gun and just start punching that guy in the face man, that is what you know!
Don't bring a weapon if you aren't going to use it. Intimidation with a weapon comes from the feeling that this person might actually use it on you. To be honest if I was in your situation I think I would have at least shot him in the leg as soon as I had the chance. One into the leg then reach around and pull the seat lever and ram yourself into it. Get his ass locked against the stearing wheel by his seat with a bullet in his thigh and punch his girlfriend in the face a couple times and you would have been stylin'