I like to use an automotive interior cleaner that doesn't leave a greasy residue behind like Armor All does. I've gotten good results out of Meguiar's products. If there is a build-up of grime, try scraping the bulk of it up first, then soak the remaining stuff in cleaner before scrubbing. Toothpicks and toothbrushes can be very useful.
As for the cowl, you could try one of those "back-to-black" products. They don't really make it look like new, but they do make a difference, if only for a few months. The cowl can get pretty brittle after a couple of decades, so be careful if you try to pull it off for painting.
1995 Mustang GT (5.0), daily driver
1995 GMC C2500 (5.7), alternate daily driver