If you want to connect an amp to your car stereo which doesn’t have a speaker level inputs then it means you need a line output converter to have a preamp outputs. So you can connect your receiver to an amp by converting your receiver’s speaker output into preamp-level signals. It consists of a transformer and (10 to 20 watts) a high-voltage resistor and connects to amp by using RCA patch cable. The amplifier in the head unit is being loaded by the resistor. It has also an adjustable gain feature to pull output.
A line out converter, or L.O.C., is a device that turns the speaker-level audio signal in a car audio system into a line-level signal that one can connect to an amplifier. This is necessary when one is adding an amplifier to a factory head unit, since factory head units do not have RCA outputs to get signals from. Typically, L.O.C.s come in two and four channel versions for a variety of installation options, and may be easily removed to return the vehicle to stock.
Line output converters (LOCs) are used to convert a head unit's speaker outputs or the outputs of an OEM amplifier into a signal suitable to drive the preamp inputs of an amplifier. Most line output converters (LOCs) used in the past few years are designed for high output head units.