To the best of my knowledge, you cannot claim the entire cost of the car as a medical expense.
However, you can certainly claim the "Upgradation Costs' as an expense. IRS Publication 502 says ".....You can include in medical expenses the cost of special hand controls and other special equipment installed in a car for the use of a person with a disability...."
You can have the full publication for your reference at http://www.irs.gov/publications/p502/ar02.html#en_US_publink1000178913
I am sure this would help...