From memory the speaker cabling goes to an amp that sits behind the glove box. You should be able to see it from the foot well. I cut the Ford connector off & wired the new head unit to that. You just need to use the power etc that went to the old Ford Stereo.
I kept the old Ford connector so that I could rewire it if I wanted to put it back in. Any good Car Stereo shop should be able to give you what you need so you don't have to chop it about.