No, it doesn't make a difference. The cost of the magnets themselves is a deductible advertising expense. But simply having advertising signage on a vehicle doesn't change a personal trip into a business trip. If being at the car show is reasonable, ordinary, and necessary expense to further your husband's business, then the mileage to and from the car show could be considered business mileage.
Publication 463 from the IRS says on page 20:
Advertising display on car. Putting display material that advertises your business on your car doesn’t change the use of your car from personal use to business use. If you use this car for commuting or other personal uses, you still can’t deduct your expenses for those uses.
This issue has also been taken to Tax Court -- where the court upheld the IRS's disallowance of a vehicle deduction claim based on the vehicle's advertising material. Another expert explained it well in this post.
In summary, traveling to and from business events may count as business mileage. That business mileage can be used to take the standard mileage rate deduction (if the vehicle qualifies) or used to prorate the total cost of the vehicle for the year into business vs. personal expenses. But having business signage on your car is not relevant to whether a trip is for business purposes or not.
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-- KimberW
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