Hey I have a 99 mystique with a pretty nice system in it...here is how I got mine off...just undo the 4 screws that are under the stock deck and above where the cup holders are. Then you just pull the thing out. Granted the other way was easier, but I decided to do it this way anyhow. Definitely get the wiring harness, and if you have the premium sound with the factory amp you are going to have some fun smile Also you are going to have to do some slight mods to get the 6x9's to work, but its not a big deal. Just take your time and do it right.

99 Mercury Mystique Red (toredor) ATX 2.5 V6. Stock right now excluding sound system.

Sound System:
Alpine CDA-7864
Infinity Kappa Components 5.25
Infinity Kappa 5x7's (rear)
2 12" JBL Power Series Subs
Rockford Fosgate 160a2 (interior)
Crunch 4004C (Sub Amp)
Phoenix Gold Power Wire, Fuse Holder, Distribution Blocks, Speaker Wire, and RCA's.

Future Mods: KKM intake
Indiglo HVACS and gauges (hopefully SVT)
SVT exhaust
Stock SVT wheels
Body Kit?
email if selling any of these items.