how to disable auto dimm or set brigthness level on JBL Horizon 2 clock display  

Press the scan button and keep it for at least 3-5 secs, make sure you are using jbl horizon 2 fm.

Hmm weird, it's work fine for me as in the video, can you please confirm you are using JBL Horizon 2 FM? You can try also resetting the device first and then try again the step in the video. Thanks.