Your '98 (Town Car) will only have a subwoofer if it's a Cartier, or a Signature with optional JBL audio system.
The subwoofer has an amplifier mounted to it.
The speakers are normal, mounted in the 6x9 openings and will be marked with the Ford JBL audio system badge on the BACK/magnet of speaker if equipped w/ JBL. Otherwise, normal paper cone 6x9s.
Put on a CLEAR station with 'easy' music, nothing too loud or bass-ey. Turn the volumn up some then simeltaneously press the 3 and 6 buttons. The radi owill enter self test mode and do a speaker walk around test.
Sub (if equipped) will also be tested.
Listen for the blown speaker and note which one it is, will save time removing/installing parts that are not defective. If a 'short' error is displayed on the screen inspect that speaker and it's wiring.
Turn radio off then back on to exit the self-test.