Im guessing its not a simple ISO plug then? your going to have to look at the speaker wires on the speakers, or run new ones to behind the dash and feed it to the ISO plugs, once again with crimps. You'll have to figure out which wire is plus and negative, have a closer inspection on the wire it might even hint which is positive and neg. To be honest though, if you need to go through the hassle of feeding wires to the dash, just feed em all to your boot and get a cheap small amp for £20-£30 the sound will be hundreds times better!
Edit: once you hear amped door speakers/components you'll never go back! its an essential for any car I buy in the future ... a small 10" sub, with amped door speakers.