Peter,

Just a guess based on late model Isuzus, but you should be able to see a black rubber hose coming up from the front diff into the engine bay. One popular spot has always been to clip it to one side of the radiator. Lay under there with a flashlight and you'll find it. Isuzu has been using vents raised up into the engine bay for years. Late model Isuzu's actually use trick hard metal lines and a special little sheltered box at the firewall.