Regardless of whether you live in the either the Seattle/Tacoma or Vancouver, Washington Metro area, you almost always finance a car during a Chapter 13 Bankruptcy. There are, however, a few hoops that you and your attorney will need to jump through.
First, both you and your Bankruptcy attorney will need to review your budget to make sure that you can afford the extra payment. If you can afford the extra payment, your attorney can simply file a Motion to Incur Debt. As long as their is a demonstrable need for the vehicle, the Washington Chapter 13 Trustee is unlikely to resist the Motion.
If you cannot afford an extra payment and you really do need another vehicle and the only way to afford it is to lower your Chapter 13 Bankruptcy Plan payment, your Seattle or Vancouver Bankruptcy Attorney will need to file a Motion to Amend Your Chapter 13 Plan in addition to a Motion to Incur Debt.