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When you buy the wiring harness adapter there's a chart included in the
package. You don't chop up the vehicle's harness.

I have a PDF somewhere if I can find it I'll send it.

If you're in ENGLAND, do you have different connectors than North
America?

'Curly'

2003 US Honda Civic Hybrid, single CD, AM-FM.
Audio Unit Connector A - 20 pin
1    -    Not Used
2    yel/red    ACC (Main Stereo Power Supply)
3    -    Not Used
4    -    Not Used
5    Pnk    Right Rear +
6    Blu/Wht    Left Rear +
7    Grn/Yel    Front Passenger's Door +
8    Grn/Blk    Driver's Door +
9    Red/Blk    Lights-on signal
10    Wht/Red    Constant Power
11    -    Not Used
12    -    Not Used
13    -    Not Used
14    -    Not Used
15    Blu/Yel    Right Rear -
16    Blu/Blk    Left Rear -
17    Gry/Red    Front Passenger's Door -
18    Lt Grn    Driver's Door -
19    Red    Dash Lights Brightness Controller
20    Bk    Ground (G503)

pin 1-10 are top left to right, looking into the back of the radio.
ping 11-20 bottom, left to right.

I'll let you decide if "Driver's Door" is a proper designation across the
pond. ;-)