Rear defroster and rear wiper.

2015 Cadillac srx. When i turn on my rear defroster,
the rear cargo light on the tail gate door turns on
and rear defroster does not work. Also when i try to
use the rear wiper (rear defroster off) the same
light turns on and wiper moves slowly and the light
gets brighter and weaker with wiper movements.
When rear defroster is on and the light turns on,
the rear wiper will not work.
Can this be a ground issue?