There really is no security. Bolt cutters, wire nuts, hammer and I think we could take any car that does not have a ECU interrupt (which I believe is what the PCM/BCM methods really do).
That said, the actual brains will all be under the hood (end state). so unless you know the key matrix well enough to guess the pass code you will need to get into the engine compartment and hotwire it there. I'm effectively removing all in cab analog (which is really just digital control v+) as well as the associated uC and replacing them with my own. There will still be some in cab uC but those will be non-critical systems.
Not sure that answers...
But as you can see, fix a simple switch problem with a rube goldberg machine.