If they are not running at the time of the EMP attack they will most likely be not be negativity affected.
That’s the effect from the E1. It is (relatively) short ranged.
The real problem is the long lines picking up the E3 energy and frying the grid.
Ted Koppel wrote a good non-fiction book about cyber attack or physical destruction of key nodes having the same effect.
I would like to think it was that simple. The problem is that EMP effects are difficult and expensive to test. I don't know of any tests that have been done for at least 15, maybe 20 years.
My guess is that a lot of cars would not be affected, and that significantly fewer cars that were not running would be affected.
But, I don't really know. The circuitry in cars has changed a lot in 15-20 years.
If you know of any testing done in the last 20 years, I would like to know of it. The last I knew was done at the facility near Chesapeake bay. (I remember visiting it, but it was a while back.)