I popped this piece off to look at something on my head unit, then when it came time to put it back on I noticed it didnt snap in, it was held on by some sort of light adhesive that looked like black rubber and it molded very thinly to the plastic piece that actually holds the head unit in.

What could I used to put it back in place, rubber cement??

Its this piece

 

Most trim pieces like that are held in by a couple of clips or tabs. For example my alpine headunit has 2 tabs on the top and 3 on the bottom which are attached to that piece. So basically it simply snaps back in although sometimes it can be tedious for whatever reason.

I'm assuming you bought your car with the headunit already installed or you had a shop put it in? If so thats probably why there was an adhesive holding it on as the tabs were probably broken off. This leaves you with two options. You can get some type of adhesive or you can purchase a new trim piece. Good luck

 

Yes it came in the car when I bought it, never had an issue either like it never rattled or fell off, so it was on there well! And when I took the piece off it simply peeled away, no clips or anything, but theres no adhesive left on the bezel so it could re-stick itself.

I might try a thin layer of rubber cement or just dot it on to each corner and press.