Upvote:4
- Do i need an Esta if i cross by land and if not what is the process?
You don't need ESTA. I don't know the process, I'm afraid, but I do think that having ESTA might allow you to remain in your car. Someone else here will surely answer on that point.
I think that another way to be able to stay in your car is to apply beforehand for an I-94 form, which costs $6 instead of $14 (and you'll have to pay $6 at the border anyway if you haven't gotten one or the other ahead of time). I believe there are plans to begin requiring ESTA for land crossings at some as-yet-undetermined point in the future, so anyone reading this much after today (January 2020) should double check.
- ... if i fly into the US and leave by land back to Mexico, and i want to come back to the states in say 6 months, will it be seen as i overstayed my first 90days on my Esta as my time is counted in Mexico?
You won't have overstayed, but your exit will probably not have been recorded. There is a process for correcting that, which you can read about at How can I ensure my exit from the US by land is recorded? on this site.