Dude, you're both 16 years old. You don't love her.
As for your argument,
Posessing weed is illegal, possessing car audio is not. The only time you can get in trouble for CA is if you are disturbing the peace, which you can do with any noise-making device
Owning car audio will not keep you from getting into any schools or jobs or school activities. Weed will.
If she needs weed enough to fight about it at such a young age, she has a problem. Weed isn't addictive, but she may be using it to drown out some personal issues that need to be addressed.