Some of the 98's have an external amp that powers the speakers. If this is the case there will be a small square wiring harness in addition to the main harness that contains your power/ground etc.
Chances are you only hooked up the one wiring harness while the other is necessary to get sound from your speakers. In addition there should also be an amp turn on that needs to be connected (blu/wht wires) to get power to that amp.
Check this product out and see if you find something that the small square plug fits into in your car. 88-97 FD/MA PREM CONNECT