Bankruptcy courts frown on people incurring new debt while their cases are active, but they understand things happen that require petitioners to take on more bills. Thus, it's possible to purchase a vehicle while you're still paying on your chapter 13 bankruptcy, but you need to do these two things first.
Create a Car-Payment Friendly Budget
The purpose of chapter 13 bankruptcy is to make your debts more manageable. So, the trustee in your case won't even consider your request unless you can show you have enough money to cover the monthly car note, required insurance, and your plan payments.