I would try the pet urine thing as PIKKA had wrote about but without spending any money I would buy some charcoal to grill with & put some of the charcoal briquettes in the car and shut the door. After about 3 to 5 days the charcoal should/will absorb the smell then just simply throw the charcoal away with the smell and all. This works in any room that someone has thrown up in too. Keep us posted on what you end up doing!
I agree with Pikka, remember that the enzyme soak (I like Natures Miracle) needs to be soaked deeply into the stain and can take days to dry before the smell will be gone.
Lemon juice and white vinegar, full strength or enzyme product for pet urine [it sort of eats any biological stain/smell. Leave it to do it's work for some days, and then use white vinegar again if needed. I'd try the enyzme first and then the vinegar since it's on fabric rather than say a floor.
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