CITROËN C3 Handbook page 175

Practical information
Connector lines
a1 (+) Right hand rear speaker
a2 (+) Right hand front speaker and tweeter
a3 (+) Left hand front speaker and tweeter
a4 (+) Left hand rear speaker
a5 (-) Right hand rear speaker
a6 (-) Right hand front speaker and tweeter
a7 (-) Left hand front speaker and tweeter
a8 (-) Left hand rear speaker
a9 not used
a10 not used
a11 not used
a12 (+) Permanent
a13 not used
a14 not used
a15 (+) Ignition switched (operation of the audio system only with the ignition on)
a16 Earth
178
Never use a separate wire to connect
the (+) of your audio equipment (risk of
discharging of the battery).