Looking at the amplifier output diagram, (radio/cd that is already fitted) you can see that neither of the speaker terminals (A and B) are connected to 0V (Car ground connection).
The speakers are connected as shown in the diagram to enable more power to be achieved from a single 12volt supply.
An RCA type plug has only two connections. The outer connection is usually 0V ground (screen connection or 'earth').
If the input of the additional power amplifier is of the un-balanced type (RCA type plug), it cannot be directly connected to the type of output shown which is typical of a car amplifier output.
Amplifier diagram of one channel.
A convertor of some description is required.
The most simple of which is the audio bal-unbal transformer.
This can be connected simply as in the second diagram, if the new power amplifier input level is rated for speaker outputs. (They usually have input level adjustment control).
The two centre connections on the transformer shown are not used.
The 22 ohm resistor (R shown on the diagram) is there to provide some load for the existing amplifier because its speaker will no longer be connected.
(Two of these will be required for a stereo amplifier).