The IRS will never buy it. Parts removed from your inventory for personal use (and installing them on your own car is personal use, any way you look at it) are labeled as "removed for personal use." What you paid for those parts are not tax deductible by any stretch of the imagination.
those parts you put on the vehicle would add to its cost basis or be repairs. so if you legitimately use the vehicle in your business, ie take a tax deduction for using the vehicle in your business, then there would be a tax deduction based on business miles to total miles. another caveat is who owns the vehicle you or the business. it won't matter if you are a single-member LLC or sole proprietorship. if your business is a partnership and it owns it, the deduction would be at the partnership level while if you own it then it could be an additional schedule E deduction. if owned by a corporation it would be a corporate deduction but if you own it then it would likely be a non-deductible employee expense. the Corp could set up an accountable expense reimbursement plan and reimburse you per business mile base on IRS vehicle mileage allowance.