Help troubleshooting AV cables

First you might want to check if your TV has a menu where you can select different inputs.

On mine I have a video switch box simply because its way too hard to get to the back panel every time I want to change inputs. It requires unbolting the TV from the shelf.

Have you gone to the television menu to set the TV to the correct set of connections? Most TV's now have 6 or more sets of connections to be used.

There's really no meter to test the jacks. A TV monitor is what's needed and you have that. I would make sure you TV is set for the correct input your hooked too. It may have 2 or 3 such inputs.
I would also make sure your head unit doesn't have a setting where you can turn on or off the second output.
Are you sure your cables are good. It would be very unusual for all 3 to be bad at the same time but you never know.
Are you sure the jacks by the TV are outputs? They may be inputs where you could connect a game system or something and play it thru the head unit.

Are you sure that in dash unit is not an INPUT rather than an OUTPUT?

Just wondering based on units I have owned in the past.

Do you know the in dash unit make and model?

Question: Are those jacks on the TV faceplate or are they on a wall plate near the TV?

Reason and true story: On my main TV they are on the TV, but there are also a bunch of jacks on the back for assorted types of input including another copy of the A/V Line in (Red/White/Yellow) and also a set of output jacks.

Well... I could not get it to work with Line in from the rear, worked fine from the front but not from the rear.. Finally I accessed the rear of the TV and this is what I found

OUT AV-1 AV-2
X . . 0 . . 0 Video
0 . . X . . 0 Audio
0 . . X . . 0 Audio

X means there was a cable plugged in.. I think you can see how I fixed it.

What does this mean for you? Are you sure the wall plate jacks are connected to the TV?

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If like me you have a "home Theater".

IF I play a source via the FRONT jacks, the TV does it all
But if I play via the factory REAR inputs (I got to fix that one of these days) the show goes first to the Home Theater and THEN on to the TV So I have to have it on as well.

You probably need to select the right input on the TV menu. Try this, turn on DVD with a DVD in it. Then with the TV on, select MENU on the TV remote and find INPUT, it should be on the remote itself. now change each INPUT one at a time until the picture shows up. there will be 4 or 5 INPUTS, you just have to find the right one. Good luck

Before I could post answers to the questions, I tried it again tonight. With everything the same, it worked. But with a lot of interference. The tv is one with a brick, which I had bypassed. It is plugged direct to 12v. So I changed it to the brick plugged into 110v, and it worked perfectly. I'm not sure what to think of that since the tv otherwise works great when plugged direct to 12v.