Oh, Ok, thanks. So that signal is coming to the PSU board from the main board so if I force it low by shorting C25 the backlight turns off. In other words, I would short it to gnd after the R42 2k2 resistor and not at the connector. Please confirm this is correct. Since I expect these components to be SMD on the lower side I might have a little trouble finding them. It seems quite easy to implement this part of disabling the backlight. The next part will be deciding how to control this and I might look at the video signal, the mouse signal, etc.
Main board generates that signal to say it wants backlight on, and power supply turns on backlight. The power supply produces both a standby 5V, which is always on, and a 12V/24V supply. The 12V/24V will be switched on when the TV needs it, primarily the higher voltage supplies are for the LCD panel, speakers, maybe an LNB amplifier if the TV has one, as well as the motherboard's supplies that don't run in standby (USB power for instance).
You would probably be okay putting a shorting relay or transistor across C25, which would allow you to pull the voltage down across C25. The resistor there limits the current that will be pulled from the system-on-chip that drives this signal. I would not recommend you drive the signal yourself.
Another question. In the schematics I notice items marked SXX (S24, S25, etc) which look like jumpers but in some places they make no sense to me as jumpers. Can you explain this? What am I missing?
Vestel make cheap televisions (among other products), and like to minimise their R&D and production costs. Consequentially, they design a PCB to accommodate possibly hundreds of options, a bit like the venerable Ford Transit can be ordered in some 1 million configurations. However, the good comparisons end there... Their power supply boards are often designed to accommodate anywhere from a 22" set to a 50" set with a few added capacitors and diodes here or there, but they always tend to be woefully undersized and are prone to failure. They use some truly funky designs. It looks like the 5V rail on your board is derived using an NPN transistor from the 12V rail, rather than an LDO, probably because it saves 1-2 cents. That means if the 5V rail shorts it probably smokes that transistor.
The 'Sxxx' stuff you see in the schematic will be for the various options. And they never give you a schematic that covers your exact model. Which can be a headache. But, at least you can get the schematics. You just have to figure out what to do with those schematics.
Edit: misread you. You got it the right way around first time.