I am confused. Are they mandating a backdoor, or is there already a backdoor, or are they allowing the police to exploit zero days? If it is the latter, it is sad that the authorities assume weak security as a given.
The baseband is an opaque binary blob that operates outside of the phone's main OS, and its contents are usually considered a trade secret by the manufacturer since it handles low-level hardware interactions with the main radios/etc.
Personally, I would be surprised if those systems weren't compromised by agreement. It's already common to see criminals and dissidents get busted because they think that turning a phone off stops it from reporting location data.