Neon idea may need help of neon sign man for your custom demands. Maybe switch in ground circuit to turn off during daytime. Neon tubes stand up to usual jolting of vehicle without problems.
The six square wave generators is our six spark plugs on a six cylinder engine. Six small high voltage capacitors connected to each spark plug terminal will light neon tubes or letters (shown as lamps) on side of panel truck. Note firing sequence of engine has to be accounted for in design. Engine running the lamps are flashing. Faster speed is faster flashing.
