Transistor in rally gauge ?

You're thinking of the IVR. (Instrument Voltage Regulator) It regulates the voltage to supply 5 bolts to the gauges. I bought all the parts needed from Radio Shack for less than $5. But that was over 10 yrs ago.
It's been awhile but I think there is a simple IC, a heat sink and one more component (resistor, capacitor, something) required.
In a 67-69 cuda the stock mechanical voltage regulator is built into the gas gauge. Pretty sure on a b body it's a stand alone assembly. I performed a vasectomy inside my gas gauge to disable the stock IVR, one snip and it was no longer operable.
I think they are available for about 30 bucks if you don't want to make your own.