stay quiet and stay out of it. if your wife brings you into it, tell her there is nothing you can do for him, or her in this regard
let her know you don't want it to negatively affect the marriage
if she is helping him with her own money she earns, you have to deal with that. it's her son, and he and her were there before you. you entered into their lives willingly, and in someway have to deal with them both. if you can't, you will have to make some tough choices
just don't let it get to "it's him or me." if it did get to that, YOU make the choice to leave or stay, but don't make her choose between you or her son in anyway emotionally, or otherwise
also no, you are not wrong. but nor is she for wanting to help. but she shouldn't help, because its just a car or truck. does he NEED it? i doubt he does. none of us really NEED those things. its a tough spot for you and her, but obviously not for him
it sounds like he has issues. but without more info its just hard to know what to say to you. did he have good parenting from his mom? what is his biological father like, and where is he in all this? was he abused, a drug addict, emotionally unstable / mentally ill, a criminal?
obviously something has gone haywire here. but what, and what is the cause?
i really don't believe in the "you're 18 now sink or swim" way of doing things for truly troubled teens. especially if the trouble stems from family problems. sooner or later things need be worked out, and it takes a family effort often times, one way or another
but no, he does not NEED a truck or car, and if it is going to burden you financially let your wife know you strongly object to it and will not contribute to the purchase. then stay out of it and / or offer some alternatives, perhaps
at the most, send him a china scoot. that'll teach 'em!