The common main board failure that results in loss of usable PWM signal is the little "gizmo" in the picture below. I have replaced the "bad" one with the "good" one a couple of times, but it has been a real bear both times. The solder seems to have a higher melting temperature than most of the other board components, and can't be removed with a hot air station without damaging the board. Has to be removed with a tip load of hot solder on the end of your iron. Resoldering is also tricky, it is very difficult to get the new solder to bind. I have no idea why. It "looks" like anything else on the board, but I've had the same issues each time I tried. However, it did fix the issue both times.
You can't just relocate the white wire. The other PWM pin gets a different PWM code from the CPU.
By the way, these "gizmos" invert the PWM signal from the main CPU. If you look at the input and output of a normal "gizmo" on an oscilloscope, the on/off cycle for about 80% of the signal is exactly the same pattern, except on/off is reversed. On a failed gizmo, the input and output are identical (or there is no output at all).
You may have noticed by now I am not much of an electronics guy. I am more of "a mechanic with a soldering iron". I know just enough to compare a working thing with a non-working thing, and figure out which part needs to be replaced. I would LOVE for someone to step in and properly identify what a "gizmo" really is.
Any way, here is a picture of what to swap if you decide to try:
View attachment 13262