I have attached a rough diagram of the wiring in my car.

Just finished installing a 4 channel amp, i previously had a new head unit and front and rear speakers installed professionally. I bought the amp to beef up the system and thought I'd take a stab at installing it. Here's how i wired the speakers: for the rears i snipped the wires that ran to them from the head unit and spliced new wire into about a foot of old wire at the speaker and ran it to the amp. For the fronts, i located where they entered the harness before attaching to the head unit, I snipped the wires there and spliced new speaker wire and ran it all the way to the back. I then attached everything to my amp, set the high pass filter for the front speakers to about 70 and adjusted no other dials. I turned on my car and only the front left speaker is putting out sound. Solutions? Please help?