The statute is here: 18 Pa CSA 6308 a) Offense defined.--A person commits a summary offense if he, being less than 21 years of age, attempts to purchase, purchases, consumes, possesses or knowingly and intentionally transports any liquor or malt or brewed beverages, as defined in section 6310.6 (relating to definitions). For the purposes of this section, it shall not be a defense that the liquor or malt or brewed beverage was consumed in a jurisdiction other than the jurisdiction where the citation for underage drinking was issued.
(b) Penalty.--In addition to the penalty imposed pursuant to section 6310.4 (relating to restriction of operating privileges), a person convicted of violating subsection (a) may be sentenced to pay a fine of not more than $500 for the second and each subsequent violation.
This would be a very difficult case to win. The traffic stop for a broken headlight is likely to be legal, even if you were profiled, the police had a reason to pull you over. If the beer was visible in the backseat the Court will call this plain view and your driver's license gave the police your age. The "I did not know it was there defense" will be hurt with the breath test showing you had some alcohol at some point.
I cannot tell you what to do with your case without meeting you and knowing all the facts. I would also need to know what the witnesses would say. Do you have priors? Each police department and each district court has its own diversion program for first time offenders (and occasionally second timers with good reasons) I can tell you in very hard to win underage cases I try to get my clients a different charge or a diversion program.
You are almost 21. Be prepared to show that you have a job or school or why you need to drive. Check the Court's website for underage drinking programs.
Call a lawyer. Some with meet with you and explain the local diversion programs.