Because there's variation, it's best to:
1. Consult your owner's manual: This is the most reliable source. The manual will have diagrams showing the location of components.
2. Use a repair manual (Chilton's or Haynes): These manuals typically have detailed diagrams and instructions specific to your car's year and engine.
3. Search online for images: Search on Google Images or YouTube for "1999 Dodge Avenger ignition coil location [2.5L or 2.7L engine]" (replace with your engine size). Look at several pictures to get a better idea.
Looking at pictures online while comparing it to your engine bay will be your best bet to locate it. Remember that safety first; disconnect the negative battery terminal before working near electrical components.