What is the difference between a high-level and low-level connection ?

High-level or low-level connections are used to connect an amplifier to a car radio. The choice of connection depends mainly on the car radio. 

• High-level connection means using the car radio's speaker outputs to transmit the sound signal to the external amplifier. This solution is preferred on original car radios, which do not have line (RCA) outputs.

• Low-level connection means using the car radio's low-level outputs (RCA). This solution is chosen on universal car radios known as "After market", because the vast majority have at least two stereo RCA outputs for connecting power amplifiers. It's a more efficient solution in terms of sound quality, especially if the cable distance between the amplifier and the car radio is long. 

Is it still possible to connect a low-level original car radio ?

Yes, there is an alternative: you need to use a converter like the Hilo V3. This solution is used for amplifiers that do not have high-level inputs.