Looks like a few have already touched on it above.
If your car has a factory amp, you are going to either need an interface harness or you will need to bypass it directly. Just hooking up an aftermarket head unit to the speaker wires, correct or not, is not going to work.
You can probably find an interface at Crutchfield or possibly even directly from a company like Metra.
In my case, I just removed the factory amp completely and have it boxed away with the factory head. (My '92 had the premium sound) Then I used a wiring diagram on a non-premium system from
www.the12volt.com and traced the wires to the premium connectors and figured it out this way. Of course, this is just all temporary for me until I finish the custom work and ALL of the factory stuff is coming out.
Please just let me know if there is anyway I can help.
Best of luck.