Normally, when the fuel and temperature gauges don't work, the regular answer is the voltage regulator device -- the only thing that these two gauges share in common. This is mainly for the benefit of someone else who peruses these posts looking for the answer to their own problem: the voltage regulation, to 10.0 volts, is necessary as a constant voltage source, because both gauges work by measuring current through a resistor (either the temp sensor or the fuel level sensor), and for consistent accuracy you need a reliably constant voltage source (the car's battery normally fluctuates too much for this) -- in I=V/R, and you want I to only vary with R, so you keep V constant.
Also note that the reason I wrote "device" [viz., voltage regulator] is that between the '70s and '80s (and beyond), they used a different system. The earlier cars used an electromechanical "vibrator" (of sorts), recognized by an aluminum can mounted behind the instrument panel; whereas the later cars used a simple, 3-terminal electronic chip (resembling a transistor in a TO-220 case).

That said, it's hard to figure exactly went wrong with your mistake with the red-white wire. That one is notorious for frying the electronics of anything it is connected to (other than its proper connection to a tach) -- this forum is full of stories of frying the clock, electronic speedometer (on the later 240s), etc. You may have simply fried the aforementioned VR chip on the back of the '84's panel -- that's where I would start first, by desodering and resoldering a new, 10.0 volt voltage regulator chip.

Good luck.