If you want a modern car, you're just going to have to accept that right now, they're all full of closed-source, black-box computer stuff. Short of going to work for the manufacturer and signing an NDA, you're never going to be able to get access to the inner workings of these things. The unfortunate truth is that these manufacturers are adding features without incorporating security from the very beginning, in an effort to have more bells and whistles than the other guys. They're getting better about security, but they still have a lot to learn.
The good news is that most of these hacks are at least somewhat mitigated. The Jeep one seems the worst, as it worked over a cellular connection from seemingly anywhere, to get into the infotainment system, and then jump to the car's actual controls from there. Chrysler was able to make some change to their network that (partially?) stopped the attack even if the individual cars were still technically vulnerable. The OnStar hack was a MITM between the mobile app and the OnStar website (due to not verifying the cert); it resulted in being able to do things to the car, but wasn't actually a vulnerability in the car itself. Most of the previous hacks require physically connecting to the OBD2 port in the car. As was stated in related posting, just as with computers, if the bad guy can break into your car and install a dongle, you're pretty much screwed anyway. Just like installing only necessary packages on a server to minimize its attack surface, you can also skip unnecessary vehicle options to reduce the chance of a vuln (though you may have varying levels of success getting a car with exactly what you want and nothing you don't).
We need these hackers to keep pointing out these flaws until the manufacturers fix them (and hopefully completely avoid the same mistake in the future). For now, it's still fairly early in the cycle with lots of learning being done. We need more isolation between the vital control systems and the trivial entertainment junk to completely remove the possibility of something like a USB stick being able to take over your engine, but for the most part these vulns are still rather limited in their application, due to the inherent limitations of actually getting linked up to your car's systems. I'm afraid it might get worse before it gets better, but at least these things seem to be getting addressed by the manufacturers, rather than just covered up.