The stock head unit does have 2 separate outlets.
- line level
and
- speaker level.
One of them is used , one is still available.
You dont have the instruction book for it ? It has the back side connections listed , in a schema/drawing.
On my car , the line level was in use. Bought a speakerline adaptor , and used the speakerlines for input for my Sub.You need to get the special keys , to take the unit out. To be able to see , what is used , and what is not.
EDIT : I presume you have the CR220 ? With additional amp in th etrunk ? Or not ?
The manual from :
www.beckerautosound.com
page 28 , back side pic.
Socket B is speakerline outputs.
Socket C1 is line out.
If one of those is not used , you can use the other for getting signal to your sub amp.
And eventough they are not RCA (speakerline ) , there is a converter available that makes them in to RCA outputs. Easy....
And if you have the lineoutput available , the connector that fits there has RCA jacks. I have one in my hands....that I dont need.