Hello XXXXXXXX,
Welcome to Just Answer.
Sorry to hear about your problem.
You need to find every hole including tiny ones; they can fit thru holes as small as a dime. The best thing to use to fill the holes is steel wool. Pack it into the holes and you should be good.....mice don't like to chew on it. Expandable spray foam in a can, can be used to help hold it in and also prevent air from leaking thru.....saving on your heating an cooling bill.
If you need further help or clarification on this answer, please do not hesitate to reply to this post.
Thanks for asking your question at Just Answer.
Best of Luck, Brian
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Architect
Licensed Architect- 12 years, L.E.E.D. AP, Former Remodeling Contractor- 10 years