for what it's worth...you can also write off the interest you pay on your equity loan...so in essence you'd be writing off your complete car payment. So for tax purposes it makes sense.
As far as the statement about your equity line and losing your house. If you lost your job and couldn't pay for your home, you sell it. When you sell it, all liens would be payed off, including your equity line. So, if this were to happen, you'd sell your home and your car would be payed for in the process. The only negative side would be losing that 40+k cost of your car in equity.
HTH!