There are several things to consider.

Older thicker metal cars do not do well with glue on or many of the paint less systems because the metal is not as soft. I have straitened a taillight enclosure on a late model Pontiac bare handed. Not gonna happen with a Dart or Cuda.

The location of a dent has a lot to do with the process to remove it. Flat or curved panel? Middle of panel is different than edge of panel or along a ridged body line.

Paint condition also effects the glue type pullers.

Do you have a picture?
What year crown vic?

Being in the door you may be able to reach it from inside and press it out with a piece of wood or hammer handle.

If the car is anything older than a 1998-ish I would roll the dice and get the kit. Eastwood is pretty good for most stuff and late model stuff bends like butter.