Can a Hacker Hit Your Car?

I will change the programming in On-Star equipped Escalades to self-start and come to a secret place 35 minutes after they have been turned off for the 134th time.

I hate to be the one to tell most doomsayers (one of my favorite pastimes) that you can't hack a car. 90% of all the "computers" are ASCI chips that can't be changed. Even if they car had programmable FPGAs I doubt many hackers are fluent in programing VHDL.
The worse case scenario is that they can use your phone and change the voice dials.

ASCI has little or nothing to do with it. What you mean is that the cars' computers are running programs in read-only-memory. Many of the operating parameters (like when to fire the carboned-up plug in cylinder 3) are in regular RAM, but they go away when the car is turned off and are recalculated when you restart.

A really good hacker might be able to convince your car that it needs a new sensor when it doesn't, but if that doesn't also go away with a restart, just disconnect the battery for a half hour and plug it back in. (You may have to reset the clock and you'll likely lose all your favourite radio stations.)

I have heard of some virus issues with navigation systems running Windows CE.

I'd suspect WinCE itself long before assuming I'd been hacked.

--Europium

I'd worry more about my wife hacking into the on star system to see where I'm at.

She doesn't even have to hack it to do that. You can call at anytime and they can tell you the vehicle location. Then just to let you know you've been busted they can have them honk the horn and flash the lights.