If you have premium sound, then you have an amp behind the glove box. The speaker wires come right into that, as do a 12v line and ground wire. The speaker wires/power to the amp is the big plug and the line in (preouts from your head unit) are the smaller shielded cable.

I don't have any experience on these cars w/o premium sound.

1999 Mystique LS V6 Alpine CDA-9807, Rear deck sub, Opt. TB, K&N RU-3530/MAF adaptor, custom heatshield, FSVT 17's, SCT 4-program chip, Eibach's/ST200's, Hella Fogs...