Get the garden hose out. Make it "rain" on your truck. Lay on the floor while looking up at the dash in the area of question. A bright flashlight helps greatly. Run the hose until you find the leak. If you feel that you've spent enough time staring at one area without results, re-adjust the hose. Water will travel from a source, across things, just to annoy your leg.
Tip: Masking tape helps hold the hose in certain spots so you don't have to hire an un-willing helper.
