It might be a moisture issue, causing the tach to short; after all, the car has probally had its share of exercise because people who own f/g/f cars usually exercise them.

Even though windows will fog under the right conditions, sometimes window fogging is caused by a leak in the car. Check the car closely for leaks. Look in the trunk and feel around the carpet area. If you find any moisture, make a new post, and describ where the moisture is.