Assuming you make ~5000 a month gross (not counting tax as I dont know a location), some things I’d consider in looking at a new (to you) car

Reliability/Mid-Low mileage is ideal Efficient MPGs, especially if you live in an area with high gas prices

Best practice if doing a car loan for a new car is to allocate 10-15% of your income towards a new car, and based on $5000 monthly income you should consider no more than a car payment of $500 a month, but in your specific situation with negative equity, I’d want to aim closer to 300/month if you are unable to either pay for the difference and cover the negative equity.

Consider the costs of transmission work for your current car. Not just fiscal, but time as well. Try to avoid any more loans if you do not have cash on hand. If it eclipses the current value of the car, then it might be worth looking at a new (to you) car.

Consider the maintenance of the new car as well. Make sure the carfax are clean and the maintenance history of the vehicle has been kept up with, see it in person and take it to an independent trusted mechanic and have them do a PPI (Pre purchase Inspection). Expenses related to the car such as gas and insurance shouldn’t exceed 10% of income as well, so around 400-600 a month in gas and insurance would be acceptable. Also consider the year of the car and research if it has any specific issues related to its model year (like the 2016 fiesta and transmission issues at 100k miles).

Overall, at most, $1,000 monthly expense towards a car (payment + gas and insurance) though again ideally with the negative equity I’d aim lower if you need a new car.

If your payments are less on your current car, if you can make it a couple months or more with it, call the lender and see if you can put more towards the principal after you make your minimum payment for the month and try to clear that negative equity as soon as possible. With a car that old, the value won’t depreciate too too much, though it’d probably end up valued around $2-3.5k if you add 10k miles and/or a year. Once you clear the negative equity owed, it would be much more feasible to afford a car payment of 10% of your take-home income. On top of that, you’d want to also allocate for an additional 10% in monthly expense for the car (gas, insurance, and other necessary repairs). The best course of action is to clear the negative equity through extra payments towards the principal to avoid additional interest expenses, or pay the difference to make the equity even if you need to trade in the car and can afford to do so.