my meter shows 12 volts but it won't light test light?

You might have 12V on an open circuit but if there is a bad connection or a broken wire with only 1 or 2 strands of copper left, a soon as you complete the connection, the amount of resistance will drop the voltage to next to nothing.

Simple Physics: Volts = Resistance x Amps. The bulb probably uses 8 watts of power and has about 5.3 ohms of resistance. It uses about 2.25 amps to light the bulbs. If there is a short in your wiring, meaning there is a split in the pathway of current, then all of your amperage is being sent back to the battery. The system will say that there is 12 volts but you need to measure the current, or amperage to see if there is any. If there is no current then there is no power. Voltage is a term that means Potential Power, not actual power.

Because voltage doesn't light the bulb, amperage does. Voltage is merely what pushes the amperage. Often times a burnt or shorted wire will show 12 volts but the instant you put a load on it that will drop out to nothing. Try testing the voltage with the test light hooked up (as a load) and se if it drops.

12 Volt Test Light

12 Volt Dome Light

Check for a ground on most of the older cars they run a constant positive voltage to the dome lights and switched the ground to turn the lights on and off.

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How old is the Battery? Check for Bad or Corroded/Damaged Grounds. {Most likely cause after 36 Years}

Ground?