So take a digital multimeter and set it to resistance and attach the + lead to the + of the old speaker. Attach the - lead to the - of the speaker. Do the same with the new speakers. If the resistance dose not match like the pic i drew the speakers won't work.
This is what I found for the 2000 mustang
I think that's what you have but its for a US mustang I think you're profile says your from Canada and the directechs I looked at didn't have anything for it.
So make sure your + speaker wire is attached to the + on the speaker and make sure none of your connections are exposed and touching bare metal.