The problem is that due to the underlying need for police cooperation in unrelated cases, no sane DA or judge would dare challenge a cop (or the police union) who can claim "oh it was a malfunction" or "oh uh the battery ran out" when it comes time for video footage to be reviewed and made public in the case of a police officer breaking the law.
My proposal: make it someone's (or an anonymous board of lawyers') job to review any and all police video footage. The police union is not allowed in any circumstances to know who is doing that job, nor are they allowed to protest. Make it federal law that every officer when on duty has no expectations of privacy, and their body cam must be on 100% of the time when on duty, no exceptions.
If you are going to work (and retire) on the public's dime, using equipment the public paid for, you must without question bow to public demands of complete transparency and full accountability. I don't give a damn what the union thinks, they are on our dime.