stereo question

[Calvin Krug]

I am in the process of installing an aftermarket stereo in my '89 
200, and I have completed the amp, head unit, and front speakers. 
I'm now searching for a pair of rear speakers, and I just thought of 
this:   The original tweeters on the rear shelf are still in place, 
and I'm wondering if I could wire these in, and then just get a 
component speaker instead of a two or three way.  And if I do this, 
would I need to get a crossover, or do these tweeters have one built 
in?

Calvin