on a standard stock radio there are three connectors, C1 is the speakers L and R front, ground, power, and dimmer. C2 is steering wheel controls, serial data for HUD, and speaker L and R rear. C1 and C2 are stacked so they appear as one connector. C3 is an 9 pin flat connector, only 3 pins are used for remote CD audio.
so for an aftermarket harness you need to connect it to match up properly. the smaller connector you don't need.
Here's the wiring
C1
1 gray LF -
2 tan LF +
3 dk grn RF -
4 lt grn RF +
5 blk/wht ground
6 gry/blk lights
7 brn/wht dimmer
8 dk grn radio On output (e.g for powered antenna)
9 yel fused acc power (RAP)
10 orn +12V fused
C2
11 blank
12 blank
13 blank
14 dk blu steering wheel inputs
15 dr grn HUD serial data
16 blank
17 lt blu RR -
18 dk blu RR +
19 yel LR -
20 brn LR +
