12v 8A dimmer will Not work

I have installed cabin lighting in my Jeep but the LEDs are just too bright at night. So, I bought this dimmer switch in hopes of an easy install. Previous to installing in the Jeep, I tested it with a LED strip and a 9v battery and it worked perfectly. However, as soon as I installed it in the car, the lights are constantly at the brightest setting and no matter what I do they will not dim... I have tried everything that I could possibly think of but no luck. What in the world am I doing wrong???

Product: https://www.amazon.com/Leegoal-LEDwholesalers-Lights-Dimming-Controller/dp/B003L4KKF2/ref=sr_1_13?ie=UTF8&qid=1510888353&sr=8-13&keywords=led+dimmer

Where is the negative lead of the cabin lights attached. Do the lights ground when they are mounted...ie one wire. Most LED dimmers control the low side (negative lead) of the lights. The lights need to have two leads, with hopefully neither of them in common with the housing. Or you'll have to mount them such as they are electrically isolated from the body/metal.

The way I have it set up, the lights are basically hardwired into the 12v socket in the dash. I just put the dimmer in-line between that connection and the LEDs themselves. But your saying that having a complete circuit going from the socket's +, to dimmer input +, to LED + and then back again is the wrong way to do it?

From the power socket in the dash, positive lead to dimmer IN+, negative lead to IN-. Positive lead of cabin lights to OUT+, negative lead to OUT-. The negative lead of the cabin lights only connects to the dimmer OUT-. If the negative lead of the cabin lights is connected also to the case (metal) of the cabin lights, and the lights are attached to ground (dash metal) the dimmer won't work. The negative lead needs to be isolated, connecting ONLY to the dimmer.

In short, this is what I currently have. And based on your instructions, it sounds as if this should work!