(mods will no doubt find an appropriate area for this thread
)
there is nothing to "crack" in ACARS. It's clear text, unencrypted. Airlines use proprietary encoding though for various messages, but that could be figured out.
All ACARS could be used for is to fool the pilots (e.g. upload a fake amended flight plan, for those who use that). It would still require pilots to take action before anything would happen.
The architecture of jeep and aircraft electronics are vastly different. One clue is the price. Jeeps are built to be reasonably affordable. Aircraft components are tested and certified to a completely different level.
A competent and trained engineer, equipped with expensive, proprietary tools, could achieve some mischief IF s/he had physical access to aircraft systems, in the avionics bay. That would be pretty easy to spot, if someone tried to get there in flight. "Hacking" things over the air is a movie plot (that's why it's so fascinating to many - including me). I'm a pilot and a former military "spook", who had access to tools and knowledge most "hackers" could only dream of. While I'm no longer "current" in that area, I've kept a keen interest in communication/IT security, and I would never say it's absolutely totally impossible to "crack" something, but in the case of aircraft, a successful attack would need to involve a fair amount of "social engineering", i.e. somehow get physical access to aircraft systems, or manipulate someone who has.