I have the same problem. ive found this info
http://www.tkk.fi/Misc/Electronics/c...s/linesum.html i have ordered the bits to make the circut, i let you know if it works.
The other way is to use an audio control unit with aux in, @£300

, or an rca switch box, but if like me , you will want them on at the same times so you can hear your gps talking.