In the trunk
My '93 W140 does the same thing. I found that the cable to the CD changer was not making a good connection. I reached under the CD changer in the trunk and jiggled the cable and it worked OK for a while. Every one in a while I have to do this. At some point I'll have to do a proper diagnosis and repair but this is what is working for me at this time. YMMV.
1992 190E 2.3(White/Navy)
1999 VW Jetta TDI(Pearl Green)
1986 SAAB 900 Base 3 door(White)
1983 Porsche 911SC(Chiffon)
1977 VW Scirocco(DiamondSilver)