Start with the KIKI and check output power on glowup. If OK isolate each glow plug and check resistance. Also since it's unlikely that you'll have an ammeter capable of 20 amps, you can just touch the + probe of your voltmeter to the tip of one of the glow plugs...........the - probe to ground...........and you should see approx. 12V during the 30 seconds immediately following the turning of the key to the #2 position

86 Isuzu Trooper TD 5 speed
82 Isuzu Diesel I-Mark
86 300 SDL MB 126
85 300SD MB 126
82 300 D MB 123