Sound system question and advice needed

[jmann.arch]

I have an '87 300 TD wagon that has had the original stereo replaced with a Kenwood CD player unit, and I know very little about car stereo's other than having to replace old speakers in the past.

I am planning to replace this system with something that allow me to add auxiliary components and play mp3's - but I am also concerned that the sound right now sort of fluctuates in quality, and I don't know what causes that.

It varies between sounding okay, and sort of sounding hollow - maybe like the high end of the scale dropping out. Is this caused by the stereo, the amp, the speakers, or the wiring? And if I am planning to replace this unit - do I need to replace the amp as well? I am assuming there is an amp unit somewhere behind the stereo because it shows up in the descriptions of the original audio system for this car. Also - I have looked at the speakers - and they "look" okay - but they might not be.

Since I have limited knowledge on this - any advice is appreciated.