Monitor Issue

I have recently installed an Audiobahn DVD (Under seat mount) with dual Pyramid 7.2" TFT headrest mounted monitors.  Also includes an fm modulator.  I have wired everything to a 12V lighter adapter to make it less permanent.  The issue that I am having is with the monitors.  If I power up one at a time, they work fine.  However, if I try to use both at the same time one of them fades in and out.  I have a standard Y RCA Cable inputing the two monitors into the video input of the dvd receiver.  At this point I am not sure if it is a power issue or possibly the RCA cable that I am using.  In case it was a power issue, I have wired one of the monitors to a separate lighter adapter and plugged into a separate socket with the same results.  Please help.

video amplifier $20-30 dollars I think.

you can also try a video switcher it has a built in amplifier and you can watch up to four things and no lines or dimming also you can try a video amplifier like previously posted also you can try a tv tuner those also help lines and diming

you need this http://www.autotoys.com/x/cust/product.php?productid=455&cat=96&page=1 or something like it. when you use a single video wire and Y it off, you lose video signal for each extra monitor you use. this amplifier will keep each video wire at the required 75 ohms2015 4Runner TRD Pro

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Actually, I have purchased a 1-4 amplifier and am waiting for it to arrive.  Hopefully, it will do the trick.  However, I really thought the DVD receiver would have enough video output to support two monitors.  Also, another strange issue is the fact that it is always the same monitor which fades even if I switch between the two connections.  Anyway, thanks for your advice.