JVC HAKD5Y Tiny Phones Kids Stereo Headphones - Yellow

Have you looked in the manual for advice ?

The number on the panel of your car stereo is not a measure of actual volume. It is just a comparative scale, with 0 being the minimum and 30 being the maximum. The volume as measured in decibels is dependent on the power level of the amplifier and impedance of the speakers. The internal amplifier puts out 22 watts per channel (50 watts peak) into 4 channels, and there are output jacks for an external amplifier(s). Product specs in link.
JVC KD R850BT

this could be an internal issue or a bad installation. To make sure unit work, combine both YELLOW AND RED and run to 12V positive. Black to 12V negative. If it doesnt not turn on, this is a service issue. If it does turn on, this is a installation issue.

Could be a voltage, volume from preamp or the wiring on the speakers is crossed

What kind of headphones and TV are you trying to make work as an combined pair? It sounds to me as though you have your head phones plugged into the speaker location on the TV which could be a small hole that shows a pic of speakers. Look on the TV for an 'mic' or 'pic of headphones' holes. If there is an plug in hole with either it states mic or shows a pic of headphones plug the headphones into that hole instead of where you have them plugged into now.

Another suggestion is to use MDI cords in which you may already be using aka white, red, yellow colored plugs. If those plugs are assembled in the slots incorrect that also could be the reason as to why the headphones turn down when you turn the volume to the TV down; which also would influence the TV volume its self to also disengage to go either louder or quieter. MDI cords should be plugged in the following pattern to be categorized as the following with each color following the next color; yellow, white, red. Now if you are using MDI cords presently do you have them plugged in front or back of the TV?

If the TV has hole ports in both front and back for MDI cords then reverse your plug ins. Example: If you have them plugged in the front instead switch all three to the back of the TV or vise versa.

Also another thing; since you are wanting to use headphones for your TV speakers instead of the TV speakers themselves usually plugging headphones in are controlled by TV volume buttons; however I am assuming that you are trying to state that when you plug headphones into the TV you still hear the volume in your ears and anyone in the TV room is also still hearing the TV even though headphones are plugged in, am I right?

With you wanting to cancel the speakers on the TV out only attempted to use TV volume for speakers and without it working means you have the MDI cord plugged into the audio which would cause the TV to power the volume for the headphones anyways. May I ask this; since I am a bit confused, alright are you wishing for the headphones to be able to be loud enough so the TV only powers them for volume when you use the TV?

I know that headphones being plugged into an TV should only keep their volume within the ear buds. Therefore I have never heard of headphones being able to be loud enough to power out room sound. Instead they power internal volume which would be within your ears. Headphones were meant to keep noise and volume from surrounding a room to block room noise out and to keep the sound for your/ an own persons only listening capability. So because of this I am confused as to how ear phones would have the ability to sound the room with audio.

I can understand that if what you are trying to say is that the speakers on the TV still output volume even with head phones plugged in which would be an issue, since head phones are to capture the volume and noise from the unit in which plays aloud- to actually silence it instead- for the individual whom requires or presumes to have themselves to be the only listener.

Maybe your headphone has broken. You can buy new one from Mubuy

Each stereo system has an internal Amplifier. if you move the volume knob and only get a very tiny difference in volume, then you internal amp is shot.

some cars have internal amps for the speakers try connecting the remote wire it's the blue wire with a little white on it worked in my moms van.. if that does not work try testing it with a speaker from your house

If the TV has stereo sound, there may be a speaker switch for internal/external speakers on the rear of the TV that may have gotten bumped by something. (kids, pets, something fell behind?) If no switch, the sound driver chip or speaker may be failed. If the speaker is the culprit, headphones would still work if there is a jack for headphones. Might be a good time to consider a new HDTV

The unit probably has resistors or a limitor curcuit to limit the amount of audiop going out on the headphones. It could probably be modifyed but it would possiblt burn out the headphones as this is a 280watt peak power unit . Its there so you dont blow out your ears.you could use some wireless self powered headphones as that would solve your problem!! Good Luck