Glow plug fuse
Mate, from memory the glow plugs use approx 25 amps and are fused at the battery in the rear luggage area. Had recent start problems on my x5d, obd revealed 5 of the 6 glow plugs blown. Cost 55 ea to replace. There is a glow plug control module on the passenger side of the engine under the exhaust manifold. Check your battery and you will see a dedicated line to the glow plug control module, if you need a guy and are in melb I can put you onto a couple. We hooked an oscilloscope onto the ones we fixed and oddly they still pull power at low rpm and even to 2500 rpm when cold and also ongoing while warming up. We figured it was an emissions control issue. We initially had a cold start delay but even with 1 glow plug working we were starting it for years. I only started to look at the issue at as the wife was unhappy. It just wound over a bit more and the common rail high pressure got it started even in a Melbourne winter. As for the preheat to my knowledge it should only kick in at 3.5 deg celsius or below and it's not a dde light per the manual but the word preheat on the LCD screen across the base of the dash on mine. It goes out after a short while. This was what I discovered two weeks ago anyway for 800 large. Hope it helps you and this it how my x5 runs. I purchased it 9 months ld, it has 124k and has been relatively trouble free.