It is under the drivers side of the dash board.
Additional info by RFXK (10/18/06) - It would help if you added the year of your vehicle. There are two flasher units on mid-1990's G20 vans. One is hard-mounted on the right-hand side of the fuse box. This is the flasher that controls the turn signals. The fuse box is on the inside wall under the driver's side of the dash, to the left of the steering column and to the right of the emergency brake. It's easy enough to see when you get down there. The second flasher is on a two wire harness hanging down about 4 inches in front of, and about 2 inches above, the fuse box. Again, it's not mounted on anything, it's just hanging down. You have to get down under the dash with a flashlight to see it. This second flasher controls the emergency flashers only. Both flashers take the same type of flasher unit.