How can I paint the tight area between my windshield and dash?

If you cannot remove the windshield, then mask everything off that you do not want to paint. Next, clean the dash and wipe it with a cleaner like alcohol. Scuff the paint area with 320 or 300 grit. Blow off any residue and then use a tack rag to clean the area to paint. Paint any tight places with an airbrush or very small HVLP sprayer. (Using one of these little sprayers will prevent too much overspray and prevent laying down too much paint in one place.) Take your time; the small sprayer will fill in the areas that are hard to get to with a regular sprayer.