You could also get one of many small Arduino-compatible boards and very easily program them to output a fixed or varied signal. For beginner purposes, the standard Servo library makes this pretty simple. Just make sure that the board is happy with the voltage you want to give it.
The cheap servo tester would be a simple off-the-shelf approach.
I appreciate the suggestion, but I’m definitely not trying to add an arduino or more boards, I’m up to about $30-$35 per truck already and I’m making 3 or 4.
Fortunately the lightbar is set up so that if it has power, it flashes and just cycles through the different modes, which is more than good enough for my current needs.
I am waiting on a servo tester and will mess around with the light bars and see if I can choose specific flash modes with a pwm signal, or if it’s just on or off, and that’s the only way the lights function.
In the meantime, I’m just pumped that rc modifications are popular enough that I was able to easily find these light bars online! The lights are like the icing on the cake!