1099 from Employer on Company Vehicle

Seeing if someone can clarify this for me.

 

My employer issued me a fleet company vehicle that I've had all year long and use to get to and from work as well as to all the assignments that I'm sent out to do. I've calculated about 70% of my travel is for work and the other 30% has been personal usage (I don't always travel back home).

My employer recently asked for a mileage report that I simply don't have nor was I made aware I was suppose to keep track of. Employer has made it clear that they will issue me a 1099 for the use of the vehicle and classified as personal mileage which is considered taxable income towards me. Keep in mind I'm a full time employee that receives a W-2 from my employer. 

I don't know if that's correct nor whether that's a good or bad thing. Can anyone clear this up for me?

Thanks!