You assume the cop didn’t know the law. Surely this isn’t the first time that the cop had ever ticketed someone for a broken taillight.
I submit that the cop had an ulterior motive for stopping the vehicle: because he knew that if he found drugs or SUSPECTED that drugs were involved, he could seize the car and any cash he found and keep them to fund the department’s JBT/SWAT squads.
If the finders of fact conclude that the mistake was honest, the evidence should be admitted.
If the finder of fact concludes that the mistake was contrived and dishonest, the evidence should be excluded. It should be determined on a case-by-case basis. If we make an arbitrary law we are fashioning yet one more remedy for all cases that lets a guilty party go in every case. That remedy is not necessary.