Help Identify Part

Purchased an OEM radio for 98 Civic LX from ebay recently identical to the link below:
http://tinyurl.com/lqbn8
Can you help identify the part viewed from the picture of the backside
of the radio..It shows a green connector with a black circle to the left of it..
The tag on the part identifies it as a "Security Control Main Unit" with a slide switch for either Cx/Dx or Hx/Lx/Ex..
My FSM doesn't indicate what this is for .
What is it and what is its' function?
Thanks,
Rich
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It is the Honda accessory security system. A dealer installed option from Honda.

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Howard wrote:

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That is SO COOL! I have one of those radios (without the alarm system) and wondered why it was shaped with that groove in the top.
That's a great find.
'Curly'
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