As far as I can remember, the tweeters split at the head unit, not at the speakers. My right tweeter (my car didn't come with tweeters, just the empty spot) had a cord running up to it, but the left one didn't. I was able to splice the two wires at the head unit for the left speaker and run them to the tweeter. My speakers worked just fine with no tweeters being plugged in. I don't think it will break the circuit. You should get a nice set of tweeters though. They REALLY improve the overall sound of the stereo.
