It could be failed glow plugs or relays, but most the time it's the glow plug module located to the right of the passenger footwell. I'm told they can be opened, and if you're semi electronics savvy you can search for popped caps or cooked resistors and possibly repair it, but most people just bypass it and wire a relay straight to the glow plugs. This works for a while but the factory glow plugs run on 6V, so they can cook themselves to death after a while if you're running straight 12V. I found NGK Y-115T1 glow plugs to be identical to factory, but are instead 12V. Unfortunately I've only found them oversees, so shipping takes a while.

For my setup, I used a motorcycle starting relay (https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1), really any high current relay will do. I also wired in an adjustable 12v "gun lock timer", (http://sirennet.com/store/product/sa...gun-lock-timer). Its just a 12v timer that lets you adjust 6-40 seconds of ON time. They're used with gun locks in law enforcement vehicles. The officer presses a button and it temporary unlocks their weapon holder for the set amount of time. I have mine set at 30 seconds. It stays on long enough to let the vehicle warm up adequately. I highly recommend one of these as I've forgotten and left my glow plugs switched on MULTIPLE times and it's always a pain. I've ruined sets of glow plugs and certainly shortened my battery life. I also highly recommend wiring the factory glow plug indicator light (in the center of the dash), to the hot side of the relay, so you know when they're on.

hth