The signal is not driving anything
I am wanting to create the signal to fool the vehicle ECU into thinking the engine is running when it isn't.
So the signal is driving an ECU.
The shape therefore is not going to matter but the amplitude needs to be above the minimum amplitude to drive the ECU, which may or may not be what you see when you look at it. The ECU will be looking at the frequency to know what speed the engine is doing.
However this is a closed loop system so the ECU will expect to see some response when it alters other parameters like the mixture from the carburettor. So just a simple fixed frequency might just whack all the other outputs high and not leave you with a system configuration that is useful.
So why do you want to trick the ECU into thinking the engine is running?