Tachometer not working when lights turned on
 

I have a 1990 Porsche 964 Cabriolet C2. When driving in daylight hours, the tach operates normally. When I turn on the parking lights, running lights or headlamps, the tach immediately shuts off & drops to zero. The instrument is still illuminated, but the tach stops working. If I turn off the lights & restart the car, everything is normal. Any help or suggestion would be appreciated.

That definitely sounds like an electrical issue most likely a grounding or wiring problem. The fact that the tach dies only when the lights are on points to a shared ground or a voltage drop affecting the tach signal.