Most of the older trucks ran this wire down the driver's side under the sill plate, and then up the rear driver's side corner of the cab. They swapped the location of the fuse box to the other side on the later trucks, so it might run along the passenger side sill plate. You can check Autozone's site under repair guides, and see if they have a wiring location illustration. They might not on a 99 model, but I know they do on the later models.