My MIL is a poster child for automotive logical fallacies.
"Any car that burns oil is worn out, I don't want to have to open the hood. I have a professional job now and opening the hood is for poor people."
"If you come over here with that car, park on the grass. I just paved my driveway and don't want an oil slick."
"Any car with xxx miles is worn out, I don't mind buying a 20 year old LeSabre if it's got low miles."
It seems in OPs situation there is steam building behind wanting/needing a truck, and the task is figuring out how to let this steam escape so a rational decision is made, even if it is in fact buying a truck. The two women involved are confused about wants vs needs, probably luckily because they've had a well-provided life experience so far. It would be good for the daughter as an entering-adulthood skill to ground herself in the expenses of everyday life, to help her better define wants vs needs. It's hard to come up with this income/expenditure baseline in the semi-dependent, nebulous world of being still in school while working part-time and living at home. (Adding a vehicle compounds this measuring problem.) And this magical time, seeing one's kid enter adulthood and start making grown-up decisions, is wonderful and memorable so one doesn't want to offend the kid and have them move out early. The mom/wife has hangups that sound like my MILs that need airing in the sunlight so it can be seen if they're reasonable, and where the line is that won't be crossed.