You would be much better off using a Parrot bluetooth kit.

Becker in NJ can perform the auxiliary input modification to your head unit. You send the radio in and they send it back with a jack installed on a pigtail out the back of the radio.

If you indeed have the factory cell phone then the left front speaker was a special dual coil speaker. One half for the radio and one half for the cell phone.

I did this on my 1998 E300. I made a post for this about 4 years ago in this sub forum.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech