HI, I am happy to try and point you in the right direction.
You can use vertex painting on the model and vertex alpha blend in the material to add the dirt material onto the car’s material. Here is some documentation that goes into more detail on that here.
You might also want to look into Physical Materials depending on how exactly you want to control the dirt on the car. You can use the information from this to add dirt to certain areas of the car based on what surface it is driving on. Here is some documentation on that.