the original dash tweets are already pre-wired with an inline cap. i believe the cutoff frequency is around 1500hz (if i rememeber correctly). thats pretty low, so just watch out on the volume. go here:

http://www.the12volt.com/caraudio/crosscalc.asp

for a good crossover calculator and more info. when you enter the info, pay attention to the second result listed...the 1st order capacitor value.

as for the bass....good luck. do everything you can to isolate the front of the speaker from the back to prevent cancellation without resticting the airspace to zero. it practically impossible. an easier, cheaper, and far tackier way to solve the problem would be to buy some 6x9 boxes from any local ghetto audio place.