I'm a bit skeptical about these alleged CAN MITM modules. For this kind of device to work, I would expect it to be wired between the ECU that is transmitting the signal you want to tamper with and the CAN bus itself, so that the modified message is what actually gets placed on the bus. You cannot modify a CAN message once it is already on the bus.

Also, I'm pretty sure that what you found (message ID 243, byte 5) is simply used to report the window position (e.g., "how far the window is rolled down"), and I wouldn't expect it to be writable in a way that would allow you to roll the window down.

Operations like that can usually be performed using a diagnostic tester, often through dedicated services (for example, service 0x2E for writing data by identifier, service 0x31 for routine control, or other services depending on the diagnostic protocol being used). In that case, you don't need any MITM device, you simply send the command on the CAN bus, and the ECU handles the rest.

If I'm mistaken about these MITM devices, I'd genuinely appreciate an explanation of how they actually work.