[Fresh Wind]

It’s certainly possible that the car was hacked, but I don’t think it was done wirelessly without prior preparation.

If someone got access to the car for a time, maybe only a few minutes, relatively simple wirelessly controllable devices could be installed that would override the driver’s control of the throttle and possibly also the electric power steering assist.

These are “drive by wire” cars in which the physical throttle is operated by an electric motor.

This would be enough to create an accident situation, and to set off the bomb that they also installed.

[Falcon4.0]

"If someone got access to the car for a time, maybe only a few minutes, relatively simple wirelessly controllable devices could be installed that would override the driver’s control of the throttle and possibly also the electric power steering assist."

Actually it's quite easier than that.
Just drop in a re-programed CPU which would be instructed to go full throttle after the car reaches, say 50 mph indicating that it is on a highway.
And from my understanding you can also disable the transmission from changing gears.

[varyouga]

It has already been proven that OnStar has a direct connection to the car's main computer (ECU). Law enforcement has already used it to shut down or limit the throttle on vehicles during chases.

In a fully “drive by wire” car, everything can theoretically be overridden by the ECU. From the ignition to steering to individual braking of each wheel. It is not much of a stretch to assume hackers can take you for a ride in one of these cars. Especially if the hackers are government and OnStar gives them the keys.

I don't know about the Mercedes version of Onstar but I'm sure it is equally capable.