Dash/Stereo Illumination (bright/dim/bright/dim)

On my 2016 the illumination on the stereo and dash cluster will go bright, then dim (but never off), then bright and dim, over and over since the first day I bought it used in 2018. There is no auto dimming on my year I believe? The truck was bone stock except for a small lift, no interior mods..where would you start? The stealership wants to charge $100ph to investigate but initially blamed me and said the computer reported I unplugged the amp on the JBL in the back, which isn't related I don't believe, but I did unplug it once.