Wheel arch bones require an opposite bone. This is so corresponding wheel arches move in opposite direction when morphed.
When the mirror script is run, it determines which bones require an opposite by checking all the vertices on the zero X axis. If none of these vertices have a weight value then the script will create an opposite bone for it.
If, after running the script, your double sided body does not have the expected opposite bone, then check that none of the vertices on the Zero X Axis has a weight applied to it for the bone in question. Remember that Zero weighted bones still count and you should use the "remove Zero Weights" button in 3dsmax.
In some cases your body may have bones, other than the wheel arch, that are purposely designed to morph vertices on the outside edge only. e.g. such as those found on the 10sSedan02 C pillar. In cases such as this the process will create an unwanted opposite bone. This can be avoided by weighting a vertex on the zero X axis to the bone in question. It best to pick a vertex that is weighted only to the root bone so as not disturb the weights of other bones. Once the body is double sided you can then remove the weight applied to that vertex.