I've used gasoline and a rag in a well-ventilated area to remove grease, grime, and dealer-applied undercoating. Just dip the rag in the gas and start rubbing. It doesn't affect paint so long as you do not have the painted part immersed in the gas for extended periods of time. I don't know how gas will affect markings, however.
There are wax/grease removing solvents for prepping sheet metal just prior to painting, but don't know how well they work for built-up grime. I have used it on exterior paint when I found road tar. It removed it quite easily.