Backlight Driver (PC Engine GT)
The backlight driver circuit that generates the high voltage (900V+) needed for the fluorescent tube to light up, is located on the motherboard (Back board that reads the game cartridge).
Bypass Front Board
The backlight circuit will not be turned on/activated without the front board being plugged in, or shorting pin 8 to ground on the LCD 12 pin connector (which is what the front board does).
Backlight Turn Off Pad
There is a pad labelled BLH that is bridged at the factory. When it is bridged the backlight circuit is enabled.
If you remove this bridge, the backlight will not turn on.
High Voltage Circuit
The final output should appear on the red wire (bottom pin) of the backlight 2 pin connector.
It should be a sine wave of 900V to 1,600V depending on load (usually 900V when loaded down with a backlight, and 1600V free swing if no load). This drives the fluorescent tube.
The main component that generates the high voltage is the T900 transformer. This is under a metal cage.
On the other side you can find the pins. The low side has approximately 30V pulses, that then transform into over 1600V when not loaded down.
Without schematics, and without diving deeper into the reverse engineering just yet, I do know it is likely the components under the transformer are involved in the backlight circuit.
The main idea will be a circuit that generates a low voltage pulse that then drives the transformers low side by pulsing the 1.2V pulse up to 30V, up to 900-1600V.
A quick probe around shows that Q902 top two pins have a 1.2V pulse on them, and the bottom left pin of Q903 also has this pulse.
It follows down to D901 which has the same signal on, but floating on top of a 5V rail on the bottom left pin.
After that I have not probed further back to where this pulse originates.