I have a 1999 Mercedes Benz C280 and my speakers are emitting a continuous high pitched sound any ideas?

if you have bose system, you can tell by the speaker grills with bose on them, than your noise is probably from the amp

Speakers can do strange things, first thing I'd do is check it with some sort of other speaker first.

Questions, is it coming from both/all speakers at the same time or is it isolated?

All speakers, it's either in the sysem itself or it you could have a partial burn/tear/rip in your harness which isn't enough to make it quit, just stop acting correctly. It could be inside your stereo itself also. Without any more info, it's hard to even attempt to trouble shoot, get and ohm meter and start from point A and run to the end.

Switch out the Justin Bieber CDs for something better.