Likely a dodgy earth connection - but struggling with instruments AND brake lights at the same time
Have you had the dash out recently?
Gradual elimination is usually the only answer - start by checking the rear light bulbs and connector, and see if there is any cable chaffing in the area.
If all looks OK, I'd next check the earth point behind the dash (4 screws to access) - it can look OK, but worth undoing, cleaning (wire brush or emery apper), applying copper grease, WD40 or silicon spray and then reassembling
Good luck!