[kbs71]

The wiring diagram shown is for a idiot light dash not a gauge dash. You don't mention which you have but it really isn't relevent. Just as to how the under dash wiring is hooked up to the gauge cluster. The 18B/W wire that runs from the 4 way splice with the 12R wires goes to the AMP meter there is also a 18B wire coming from the junction block at the battery. They both have short, I think 2 amp fuses in them. Like I said they go to the AMP gauge on the Gauge cluster or to the GEN light on the lights cluster. If one of these fuses is blown, you will get the GEN light on all the time.
Also the 16W and the 16DBL will only power up when the altenator is charging, so if you alt isn't charging they shouldn't have power.

I know this is kinda long, but I'm just trying to explain how the wiring works.

If you follow your 12R wire after it leaves the back of the firewall connector under the dash, it goes to another 4 way splice. You may have a problem there. If not I would bet the start switch is bad. That's the only thing all these items have in common. I've seen it happen before.
Most of the grounds for these trucks come from the motors or whatever the switch goes to. The wiper motor is grounded thru the case, the heater fan is grounded thru the case etc.
Do your brake lights work? If they do, you probably don't have a bad ground for the turn signals, as they would be the same. Like I said, I'd try a start switch, and I'd bet that's your problem. If you look you have a 12BRN wire off the back of the start switch that goes to the fuse block and powers the wipers, turn signals and the heater switch.

So to sum it up, change the start switch, because I believe it isn't powering the fuse block for those three items and check those inline fuses in the 18B and 18B/W wires. They are located @ a foot from the terminal and the splice.

Hope this works, if not, let me scatch my head some more over this.