It should be under the dash just to the right of the parking brake as far as I know. Haven’t had the need to locate it on my Challenger but used it on my Charger previously and I recall it was hard to see unless you looked up from below or felt around really carefully.

 

It is under the dash on the drivers side, it's really hard to find but it is there. Look way under kind of to your left if you are sitting, maybe above your shin bone area.