Yes, it sounds like they did not have reasonable suspicion under the fourth amendment, particularly if you can demonstrate that they stopped you under false pretenses -- ie, tail light was not out. Probably hard to prove, and going to end up your word vs theirs. They tend to win these.

However, if you can demonstrate that there was no reasonable suspicion to stop the vehicle, then the stop was 'unreasonable under the fourth amendment', and then anything that comes from that stop is 'fruit of the poisonous tree' and will be suppressed in court. This would include any breath tests, sobriety tests, etc. Going to be tough though. I'm not sure how you demonstrate that your tail light was not out. You will lose a your word vs theirs fight.