What is this for, see pictures

it's a stereo headset jack......

got's to listen to some tunes as you resting, duntcha know....

but just to be on the safe side, turn the key on, wet yer finger and stick it in there.

if you get zapped, then you had a Frank Zappa 8 track in the player

if you do not get zapped, then you have a Miley Cyrus CD in the player....really quite simple..

 

That’s a ground module for Zeker tong!
Seriously

 

Looks like a bathroom glory hole to me.

 

Sleeper cam lens.

 

That's a 1/4" jack.
Plug a set of stereo headphones into it, see what happens.
If that don't work, plug a speaker into it and see what happens.
If that don't work, plug a guitar into it and see what happens.
If that don't work or your truck catches fire, ignore this post.

 

I still use them. Both 1/4 and 1/8th. I like the 1/4 for it's ability to carry power in the music. My headphones can take either one. A pair of OneOdios that I snagged on sale a year or so ago. I had to burn them in against some of the pumped up you tube bass burning test music tracks. If you smell burning plastic, you are not enough headset and will burn them out.

As my hearing declines (Im already 120 db plus in both) I boost the power.