1999 Mercedes C280 Car Radio Wiring diagram??????????Please help?

It is most likely bc you have multiple speakers (mids and separate tweets) and if there is any crossover inline on the tweeter... you wont be able to " read " the ohms to know its there. But that is most likely what it is . you can try to check it with a multimeter , or use a AA or AAA (no 9V or other) battery to" tap" a sound out of the speaker by putting the neg to the neg of the batt. and tapping the pos. to the other side fairly quickly. May need help hearing with the tweets. best would be to take it to a pro and let them figure it out.