The HDMI hook-up on the back of the GT-3 is an out jack. It will send digital video and audio from the GT-3 to your TV. So in my case it only sends a signal when I use the DVD player.
I did and I suggest you leave the existing RCA cables hooked up so if your watching regular TV or hook up something else to your TV(Blu-Ray, game system, etc.) you could use your surround sound system. You need the RCA's from the TV audio "OUT" to the GT-3 audio "IN" for this to work.
It's been awhile since I looked at the back of the TV, I think you could remove the RCA's from the OUT on the GT-3 to the video IN on the TV. But I just left them there. I doesn't hurt anything...
Russ & Kim
2014 Alpine 3600RS
2007 Dodge Ram 3500 5.9 4X4
2008 Harley Davidson Road King 105th Anniversary
(2011-5 bad year)(2012-40)(2013-45)(2014-23)