MP3 player to car headunit built in amp

How can I connect my mp3 player to the built in amp on my cars head unit.  I suppose i need to do some soldering but I dont know what I am looking for.  If I connect the MP3 player to the speaker outputs it plays too softly.  I need to get it going through the amp in order to play loud enough.
How Can I do this?

Re-design6 years ago
In "most" tube amps you can add an aux. tap at the vol. control. But most likely your unit is digital and that option is not available anymore. Does your unit have FM radio?
kokaine (author)  Re-design6 years ago
Yes the unit does have an FM radio.
Depends on the head unit. Is there no ancillary input ?
What head unit have you got ?