ML320 Radio Line-Level Input

I've got an '01 ML320 and a portable MP3 player. Problem is, my 320 is Navi equipped so I can't use the standard cassette adapter with the tape deck (navi screen won't close). I tried the higher-end FM transmitters with it but the FM signal is very noisy on all stations. Does anyone have any ideas on how I could wide this into the headunit permanently.

My 2 thoughts:

Find out if the 6-disc changer in the trunk has line-level inputs and tap into that with a switch to go between the MP3 player or the CD

OR

I noticed a very small opening under the hinge piece of the LCD on the drivers side...I'm thinking if I cut the cassette adapter connector off, slide the wire thru and solder a new connector on, I might be able to leave the cassette adapter in the deck and it the navi screen will still close.

Thanks!

Don't cut anything unless u are sure, secondly fm transmitters are notorious for that, try using 89.9 fm frequency , that's what worked for me, or buy a new deck with gps nav in it