[SamAdams76]

Here's a dealer photo of a new Escalade. Notice that the mirror does not have that protrusion that we see above the mirror on Jenner's car. I can't think of any 'add-on' to a rear-view mirror that would make it look like that.

Therefore it's plausible to me that the protrusion we see could be the top of a cell phone. In fact, it looks to me like it could be an iPhone 6 Plus. It's not conclusive to me that the protrusion is on the outside of the car.

[Kipp]

I think you’re right about Jenner holding a cell phone. It’s clearly on the inside as the edge of the glass obscures the side of the phone.

[GOYAKLA]

Enlarging the photo, please note the edge/line of the glass window seemly being in front of/over the object in question.
OMHO

[fatima]

Your right.He’s holding a cell phone.