1991 chevy s10

It doesn't need an amp, it has one built into the radio. You may have it wired wrong. Please tell me what color the wires are coming from the radio that you have connected to the speakers. Each channel has it's own color, with the black stripe wire being the negative for each channel. Example: the gray wire is positive for front left speaker and the grey with a black stripe is the negative for the front left. Make sure that none of your speaker wires are grounded to the chassis.