Hmmm! All THREE warning lamps are out...that would point to the common +12V Ignition feed to the lamp/clock cluster. It's a black wire, number 15, and fused by S11 (8 Amp), at least in the US 1973 412 version.
BTW, if you need a wiring diagram to figure things out, go to:
http://www.vintagebus.com/wiring/index.html
Scroll down to the "411/412" section, and select your year and type (US or not). The diagrams are split into multiple pages, so you may have to go down the line to find the page with the alt lamp circuit. If yours is a US 1974, then this is the page of interest...
http://www.vintagebus.com/wiring/Type_4 ... 1973-2.jpg
The legend is Page 5, BTW. Also, some of the diagrams (like this one) have a German legend, so you may have to use Babelfish to translate.
The Alt lamp is K2. A black wire from S11 to the gauge cluster feeds K2, K3, and K5, the Alt, Oil Pressure, and Blinker lamps, and the fuel gauge, and another black wire from S11 feeds the back-up lamps, and the Kickdown solenoid. A green wire from S11 feeds the turn signal blinker relay through the emergency flasher switch. Funny thing, on the '74, the brake warn lamp is not fed by the same line from S11...hmmm, on my '73 they're on the same fuse. Oh well.
If S11's blown, you get none of the above. If it's just the line to the gauge cluster, you get no Alt, Oil Press, or Blinker lamps on the gauge cluster, and no fuel gauge function. It's possible also that the connector which feeds K2 and K5 is off or bad, affecting only those two. I've got both the '72 and earlier and the '73 and later right gauge clusters in the attic, I'll go fish them out and post images of the backsides for further discussion. Note that the 4-lamp (Blinker, Oil, Alt, Beam) '73 and later cluster has a thermal fuel gauge with voltage regulator, like most US cars, and the 6-lamp (two blinker, Oil, Alt, Beam, and Parking Lamp lights) '72 and earlier cluster has a magnetic fuel gauge and NO voltage regulator, like older VW's and GM products.