I can't find anything on the IRS website specifically related to vehicle modifications. But there's tons of stuff on primary residence modifications for medical necessity. I would therefore assume the same rules would apply to a vehicle that is owned by the person needing such modifications so they can drive it.
So what I see on the IRS website says that the cost of the modifications are tax deductible, not the cost of the entire home. So replacing the word "home" with "car" or "vehicle" I would assume the same applies. Note the IRS pub 502 says "deduct", and as far as I can find does not even insinuate that anything has to be amortized.
However, on several other tax websites that are not "official" IRS sites, they say that modifications to the vehicle have to be amortized over 10 years. Thing is, I find nothing in IRS Pub 502 which states that. In fact, I searched that whole document for the words "amortize", "capitalize" and "depreciate" and those words exist nowhere in that document. But the word "deduct" is all over the place.
