Car amplifiers are equipped with one of two types of inputs--RCA or speaker wire. In the former, the signal from the receiver is carried to the amplifier through a cable with RCA plugs on either end. In the latter, a length of speaker wire is connected to the amplifier's speaker inputs, typically to a set of screw terminals. You can combine the two by connecting a receiver's RCA outputs to the amplifier's speaker inputs through a simple wire adaptor.
Purchase an RCA patch cable of sufficient length to reach from your receiver's RCA outputs to the speaker inputs of your amplifier. The length you need will vary, depending upon the location of your receiver and amplifier.
Cut off the RCA plugs on one end of the cable with a pair of wire cutters.
Separate the positive and negative leads of the cable from each other at the cut end and strip 1/2 inch of insulation from the ends of the leads.
Plug the end of the cable with the RCA connectors into your receiver and route the cable to the amplifier location.
Loosen the screw terminals of the amplifier with a Phillips screwdriver and slide the wire underneath the terminals. Tighten the screws to hold the wires.