Does a 1998 Chevrolet Cavalier have an auxiliary input jack?

No, a 1998 Chevrolet Cavalier did not come standard with an auxiliary input jack. Factory car stereos from that era generally only had options for cassette tapes and AM/FM radio. To play music from an external device like an MP3 player or phone, you'd need an aftermarket solution such as a cassette adapter or a completely new head unit.