A line output convertor is going to convert the sound from one of your car stereos speakers to a signal that an amplifier can understand. Generally speaking you should consider adding an amplifier whenever the clarity andor volume inst to your liking.

Pac Connecting Cars And Technology Your Total Installation
Adding amplifier to car stereo. The system depicted includes new speakers an aftermarket receiver a 4 channel amp for the front and rear pairs of full range speakers and a mono amp for a subwoofer. Simply put whenever you want. The best way to connect it to a factory system is to tap into the stereos speaker outputs for the amps input signal. This wiring diagram shows how a full blown car audio system upgrade gets wired in a car. Do not bolt your amplifier directly to your cars metal chassis thats inviting noise problems like ground loops which hum or buzz. Usually youll be installing a 4 channel amplifier with speaker level inputs.
The next thing to consider. A dual amplifier wiring kit. The extra gear youll need for wiring the amps includes. Then send the amps outputs back to the stereos harness and on to the speakers through the factory wiring. The amp can also interfere with the stereos amfm reception. You can also consider adding a subwoofer amplifier to your system paired with a nice subwoofer to bring the bass presence up to your liking.
When do you need to add an amplifier to your factory system. This will prevent electrical noise from entering your system and spoiling your music. Run these wires the turn on wire and the rca cable down the opposite side of the car from the power wire. Adding an amplifier will allow you to take control of your sound without having to purchase a brand new stereo player. If youre fitting subwoofer speakers in a box mounting the amplifier on the side of the box is a good option. All of this assumes you have an aftermarket stereo with preamp outputs.
Mount them up high to avoid damage and maximize airflow. Often times amplifiers come with this part built in but. The amp should be at least three feet from the stereo to avoid noise radiated from the vehicles electrical system. Also an amplifier will give you the flexibility to addremove different channels to your cars system whenever you decide that you want a change. Plug the rca cables into the appropriate rca outputs on your stereo.