Sounds like it might possibly be theft protection. Once disconnected
you need to put in a code, and you need to do this even if the battery
dies in the car !

This is another case of something that is supposed to be good for us.
If your radio gets stolen the crooks get nothing, but your insurance
pays out $500 or so and raises yor rates. See how good it is for us ?
Of course now you might be able to get parts from these stolen units,
but Murphy's law might kick in and give you a bad transmission. In
younger days I changed many trannies with $60 replacements from a
boneyard, but I 'm sure that whatever changed that was something that
was "good for us".

JURB