Will opening a joint account help me obtain a good loan?

See, lenders will consider the median score for each couple after collecting both of your score from the three credit bureaus.

For example, If your wife’s credit score is below 620, and your score is higher, then you will not qualify for a better term. To get better interest rate, both of your median scores should be higher(740 or above).
What is your current credit score?

It will help if both of your score is more than 700. It will definitely help if your husband's score is more than 800.
If your score is below a satisfactory point, then if possible, it's better you raise your score to some extent and then take out an auto loan. It will take some time but it'll help you to obtain a loan with suitable T&C. So, it'll also help you to manage your loan properly.

Could you please tell us both of your credit scores? Actually combining your finances will work only if both of yours credit score is above 700.