Required taxonomy term fields for node publishing?
This is a workflow requirement that can be handled by Rules. 


Set your user car type set to unpublished by default. 
Create a rule for the event Before saving content of type User Car (or whatever your node type is called)
Add a Data value is empty condition. On the Data selector you'll want something like node:field-MYTERM_FIELD:0:field-MYREQUIRED_FIELD (it depends on your field names). Make sure to check the Negate checkbox so that the rule is triggered when the taxonomy field IS NOT empty.
Add a Publish Content action.