this is from IRS PUB 502 Medical expenses
Car
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.
Special design. You can include in medical expenses
the difference between the cost of a regular car and a car
specially designed to hold a wheelchair.
Cost of operation. The includible costs of using a car for
medical reasons are explained under Transportation,
See IRS PUB 502 https://www.irs.gov/pub/irs-pdf/p502.pdf
I would also say the full cost is deductible as a medical expense in year the conversion is complete OR the year of purchase if ready-built.
