haha very clever
i did exactly the same thing the first time i wired my front speakers into my amp. i did a quick test to see if they were wired in parallel or series.
firstly you
must disconnect the speaker, tweeter and wiring connector on the stereo. then you have 3 pairs of terminals to do continuity tests on. if the positive terminal for front left on the wiring connector has continuity to
both the positive on the front left tweeter and the positive on the front left speaker then they are wired in parallel. if not they must be series. i wont tell you result, its more fun to do it yourself.
as you are probably aware, when in series load impedance increases – the more speakers, the higher the impedance. so if they are in series the output will be halfed between the 2 speakers.
load impedance drops when speakers are wired in parallel. the more speakers you wire in, the lower the impedance. so if they are in parallel the output will be doubled across the 2 speakers.
either way you're better off using passive crossovers to split the output between each speaker and tweeter, keeping the original wiring only on the door speaker, and adding a new wire to the tweeter. then send the signal to each crossover using the original rear speaker wiring if you prefer to be lazy.