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The deltaV needed to get from Mars to the sun would require fuel several orders of magnitude more than is curently being used, and several orders of magnitude more mass than the car.*
Have the car orbit Mars for a few hundred years, to then become a monument to man's endeavours / folly.
Or spunk thousands of tonnes of fuel into space (kinda littering it a bit) to drop it into the Sun.
* and no, I've not done the maths
It was a test, they'd normally send up a one ton slab of concrete, I think this is a bit more fun, don't you? And why wouldn't he use the opportunity to market his other business?