Here is a way to clean dirty models that worked for me on a dozen of well built and detailed cars that I acquired. These cars
were more than a bit dusty, they had been on an open shelf for 20 or more years.
First fill kitchen sink with a couple inches of warm soapy water. Use soft 1 inch or so wide soft paint brush to pre soak then use same brush to lightly scrub complete model reaching into all cracks and crevices. Don't be afraid to get it wet, as far as I know plastic models are water-proof. Smaller brushes are required to reach into interior. Rinse well under running water or use vegetable sprayer nozzle. Really dirty cars may require a second go-around. Allow to dry then apply a coat of wax.
The cars that I did came out as if they had been finished yesterday. 100% clean
And get this: Flocking survived too!
Again, don't be afraid to get them wet.
Keep an eye out for tiny parts that may come off and try to sneak into drain. Only one spot-light came off on 12 cars for me
This process worked very well for me.
Good luck