Been several months since I've done this, but let me try to remember what I did. Hope it helps.
You're going to have to take the stereo out of its cabinet. Not a big problem, just a few screws, and the unit is very lightweight.
Behind it is a forest of wires. But there should be an unused set of plugs labeled AUX1 or AUX2 or maybe just AUX on a paper tag wrapped around the bundle of cables coming from the back of the stereo. Probably some combo of white, yellow and red plugs.
Look on the back of your TV and find similarly colored plugs. They send the left and right channel audio and the video out from your TV. (you don't actually have to worry about the video plug) Just run cables with RCA plugs from the back of the TV to those unused plugs of similar colors coming from the back of your stereo unit.
Screw the stereo back into its cabinet and you should be good to go.
When you use the TV, in order for it to play through the stereo system, you'll need to set the stereo system's source to AUX1 or AUX2 or AUX (depending on how those unused cables were labeled), as opposed to the FM band or the weather band or whatever. And you'll want to turn down the sound on the TV itself.
Dennis and Kate
and Aggie the Gordon setter
2011 Cruiser Patriot 325CK
2011 Ram 2500 diesel