1989 Ram 50 2.0 - Fuel & Temp Gauges drop off when lights on.

So I finally got my truck "finished". So I drove it to work today, and half way to work I was pushed into a panic as the temperature gauge dropped to 1/8" above the mark above cold, and then my fuel gauge started to work its way fast towards empty.

Of course I'm looking in my mirrors looking for what should be a massive fuel and coolant leak. But I had a little light bulb moment and turned the lights off, and bam the gauges immediately began to rise back to operational positions.

I am going to check the fuses at lunch then test the alternator and battery tonight, as well I have new marker bulbs/enclosures, signal bulbs/enclosures, as well as replacement LED 1157 bulbs for sockets that will accept them.

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Lets try right side up this time:

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I'll put money on bad ground leads. Go through all the grounds on the engine, chassis and cab. Start with the main negative cable off the battery and work your way back. Check that the main negative isn't corroded through anywhere. Disconnect the ground anchor bolts and give the bolts and cable eyelets a big clean and polish up with fine steel wool. Bad/dirty grounds cause all kinds of grief.

Turns out the alternator was putting out just a bit of voltage and sorta keeping the battery charged, so I have a new one on order.

Well, it wasn't the alternator.
* The ground cable battery end was partially broken, so I re-ended the cable and cleaned the other end at the block.
* Still when I start the truck bring it to ~2k rpm the volt meter still reads only like 12.2 12.3 and rising but if the alternator was properly running it should read 14.2 should it not or will the voltage read lower if the battery is charging?
* Also if I pull one of the cables off the the battery while the truck is running, if the alternator is working the truck should stay running correct?

I also have an extra wire that runs with the alternator wiring it is solid yellow and about the same gauge as the two that plug into the back of the alternator, I'll try to post pictures tonight, I have no idea where this wire goes or if it is part of the charging system.

Thanks in advance for any suggestions on this issue.

The output voltage will be more around the 13.6-13.8v mark with the alternator working properly. The truck will stay running with the battery temporarily disconnected but this is really bad on the alternator. The battery is an integral part of the charging system and taking it out of the loop loads the crap out of the alt.