I was purchasing a car last year, and within 90 days returned it, because it had more mechanical issues than the car was worth. Can I still use it as a deduction?

It depends on how you want to use the deduction. 
If it was a business vehicle, you can enter the dates of service, amount paid, and amount refunded. 
If it was for the sales tax paid, and 3that was not refunded, then you can still use the sales tax toward your itemized deduction. In most cases people use state and local taxes paid, rather than sales tax, but if you are in a state without individual income tax, you might be using sales tax as the alternative. 
If it was not a business vehicle, and you are using the standard deduction this year, there will not be a place to use the vehicle as a deduction.