Hi Lohliak,
I also have that remote, that is for the rear screen in the back. The one I pictured above is separate remote for the digital TV tuner. Your Alphard may not have the digital TV tuner built in as it was not a standard feature. I think mine was installed by Toyota a few years after the car was built but I don't know and it may have been an option from the factory. The only built in TV tuner is the one for Japanese analogue TV. I only know mine had it fitted as I had this extra remote and the manual for the tuner included when the car was imported. On the left of my steering wheel you can also see an extra IR remote receiver which I assume is used by the digital tuner.
If you do not have the digital tuner fitted the RCA connectors in the rear should just go to the Video channel on the head unit. Do you get a video signal if you plug in something like a separate DVD player, or is just the audio not coming through? If you get video but no audio that may mean the audio connection is faulty.
Rojie for the antenna I assume the one for the digital TV tuner is the ones on the front and rear window that say "DIGITAL" on them. I don't think Ill ever be able to use this tuner for UK TV as Freeview (DVB-T) and Japanese Digital TV (ISDB-T) are not compatible so these antennas will never be used again. All this will ever be for me is a passthrough for the rear RCA connectors. Slightly annoying I have to turn it on every time I want to use it but not the end of the world. If I remove the tuner the rear connectors may just work but it'll be more work than its worth most likely