Do you think there is a way, somehow, that I can use the .yft of that other vehicle as a model for mine?
Not sure exactly what you mean here? You could open the texture & copy & paste parts into your texture but generally without editing a '.ytd' ain't going to work on another vehicle.
all I have is a google image of what it looks like
It's easy enough to copy details from a google image & paste them into a vehicle's texture. Might need a bit of work to look the best in-game, but it's a good, quick shortcut, compared to creating a police logo yourself from scratch etc
The problem is that I don't have the texture of the vehicle I want
There's no quicker way to do it unless you have a template