Setting up object properties
Collision ID's must be assigned in polygon mode under Properties > General > External State > ID and are generally 122 for the front windscreen, 121 for the rear windscreen, and 120 for the side/door windows and lightbar lenses.
Next, under the General > External State > Default Compatibility field, flags 3 and 8 should always be checked as shown below.
Navigate to Properties > Mesh > Vertices > Format and replicate the fields as shown in the below image.
(be sure to click the "apply" button in the bottom right corner after each edit otherwise your alterations won't be saved)
Setting up the user-defined options
Other than the "E" and "Mass" fields, an additional "Crash"(with no assigned value) field must be added too to the object's properties.
Additionally, a common mistake is not to add the "Crash" field to the user-defined options of the materials assigned to that glass object too.
Naming of glass objects & layers
Glass should always have an outer and inner layer, the outer laying being assigned to a normal vehicle_vehglass shader and the inner being assigned to a vehicle_vehglass_inner shader. This inner layer is merely a copy of the outer layer with the normals flipped towards the inside of the model. In terms of naming though, I recommend that you name your glass parts according to the names used on generic models. For lightbars, make use of "siren_glass1", "siren_glass2", and "siren_glass3" with their respective "siren_glass# [COL]" meshes compound under the parent.