Can I write off a car on a 1099 and W-2? Or can I only write it off on one of them.

[KarenM90]

Unreimbursed auto expenses are no longer deductible except for military and a few other limited exceptions. Please see this link for more details:  Unreimbursed Employee Business Expenses.

Self Employed Individuals can deduct Auto Expenses and the 179 Expensing election is allowed for certain business vehicles.

 In general Heavy Vehicles qualify for a 100% first-year bonus depreciation, while expensing for passenger vehicles is more limited.  

TurboTax will walk you through the applicable deduction when you enter the asset in the Self Employed section.