Ghost Rigger, we're arguing whether five plus one is six or half a dozen. It's kind of pointless. My final thoughts on the topics we've been discussing:
If you want a vehicle to be remotely hackproof: turning wireless off works. As does installing the EM shielding modification. There are pros and cons for either.
Wireless off: it's free, is quickly reversible, isn't subject to losing hacking protection if a door opens or if a window is shot out.
EM Shielding: stops potential tracking signals originating from inside the vehicle, not AS easy to Data Tap (requires power tools to drill a hole thru the body or undercarriage, more time/access, etc)
It's a matter of my personal opinion that the costs involved in EM shielding render it unnecessary given a free option already exists for the same hacking protection. EM shielding is only worth its cost for the purpose holding outgoing signals from going out, as far as I'm concerned. If you don't agree, we don't agree. Opinions tend to vary, and apparently ours do on this topic.
The consequences of being "dark" on gridguide: There are no rules for it because it's inherently locale-dependent. Even in the same sprawl, the consequences for not being visible on GridGuide vary from Seattle's Downtown core and Redmond Barrens. As far as I'm concerned, it's as inconceivable that it'd be permissible to drive on public, gridlink-equipped roadways without an active gridlink as it would be to drive at night with your headlights off. For all the same reasons. That cars "should" be able to avoid non-gridlink equipped hazards doesn't excuse a vehicle from the obligation to be on gridlink. Except of course in places where it'd make sense that the law/cops don't care (rural areas, obviously places that don't have active Gridguide, etc)
Look at the example on pg 260 where the shadowrunner flips a Yooie on the 502. The Shadowrunner told gridguide she wanted to make a U-turn, and gridguide moved traffic out of her way to accomodate her desire. That only works when the other cars on the 502 are on autopilot working with gridguide, but it's the sort of every-day occurance that sheds light on how out of the ordinary it'd be for your vehicle to NOT be on gridguide. If it proves nothing else, this example shows that cutting your car off from the Matrix should be resulting in tougher thresholds for vehicle tests since you can't ask Gridguide to get people the frag out of your way. But ultimately, it's all GM dependent.