NAC: how to install an input jack on an OEM radio?

[Louis-Alain Richard]

People of all solutions, I have a project :

Is there an easy trick to plug an iPod to a recent OEM radio ? I don't want
the FM modulator, neither the cassette deck adaptor. 

What I am seeking is a way to plug directly the iPod to the hardware in the
radio, even if this destroys the cassette deck.

For info, the two subjects are 2 Swedish wagons that both have a big 2-din
sized cassette+CDplayer radio. One is a 1998 Volvo, and the other is a 2001
Saab. 

Thanks in advance,

Louis-Alain