99 mustang GT new H/U

Ok, so I have a new Alpine head unit I installed in my 99 Mustang GT. Used a harness and everything was going fine until I hooked up the left front speaker wires. Once that was connected, I got no sound at all, but if I disconnected it, the other three speakers would work just fine. I am under the impression there is an amp in the factory setup, and I am also presuming the amp is the culprit. This is not a Mach system. My question is, how many amps does a stock system have for my make/model and where are they located? Also, does that sound like the amp to anyone else or could it be the headunit? Thanks in advance

have you replaced the door speakers? may be a short in that speaker.

Yeah, the door speakers are fine