Issue with blacked-out windows on vehicle model

I have installed a custom vehicle model in the POLICE3 slot. It's the fully marked Taurus from the RCMP mega pack by Walters.  The issue only seems to be with this vehicle model and the slicktop Taurus in the POLICE4 slot which is based off of this same model.

EDIT: I've actually confirmed this also happens with other custom vehicle models (all from that RCMP Mega Pack since that is all I have installed).  So maybe it is a larger issue?

Basically what is happening is after driving one of these vehicles for a while, the rear window gets blacked out so you can no longer see the rear deck lights, whether they're turned on or not.  If I respawn the car from the F4 console, sometimes it spawns with the blacked out rear window too.  I can keep respawning it until one spawns with a transparent rear window.  Sometimes it takes 4-5 spawns.  I've never noticed the exact timing when this happens, I just all of a sudden notice that my rear deck lights aren't showing.  This leads me to believe it is happening while I'm away from the car (t-stop, foot pursuit, etc.)

See attached image for an example.  The vehicle on the right has the blacked out rear window (notice you can't see the rear deck lights or anything inside the car).  The vehicle on the left is freshly spawned with a transparent rear window.

NOTE: I have applied a custom texture to this (you'll notice it's not RCMP), but this issue existed before I ever did that.

Any ideas how to fix this?  I'm not familiar with vehicle modding, but I'd be willing to learn enough to try to fix this.

Thanks in advance for any suggestions.  Obviously this is a real officer safety issue having no rear deck lights. 😄

EDIT: Adding a better image during daylight and at a different angle...

Edited August 16, 20196 yr by wrps
broadened scope of issue