mysterious box by rearview mirror

[kobalt]

What is the function of the black box on the right side of the rearview mirror? My car does not have onstar, do I need it?

[omegaic]

This box is for the rain sensitive wipers.

Rainsense
Wipers (If Equipped)
The moisture sensor is mounted on the passenger’s
interior side of the windshield behind the rearview
mirror and is used to automatically operate the wipers
by monitoring the amount of moisture that is on
the windshield.
The Rainsense system can be activated by moving the
wiper lever to INT. Set the wiper control band to one
of the five sensitivity settings within the delay area.
The bottom INT ADJ position is the lowest sensitivity
setting. This allows more water to collect on the
windshield between wipes. You can increase the
sensitivity of the system and the frequency of wipes
by turning the INT ADJ band up towards the wider
lines. A single wipe will occur each time you turn the
wiper control band to a higher sensitivity setting.
The Rainsense wipers operate in a delay mode as well
as a continuous low or high speed depending on the
amount of moisture on the windshield and the sensitivity
setting. The MIST and “wash” cycles operate as normal
and are not affected by the Rainsense function.
NOTICE:
The wipers must be turned off when going
through a car wash to avoid damage.
It is important to note that the Rainsense delay wiping
feature can be overridden at any time by manually
changing the wiper control to LO or HI speed.
NOTICE:
Do not place stickers or other items on the
exterior glass surface directly in front of the
rainsensor. Doing this could cause the rainsensor
to malfunction.

[Aurora40]

What he said. On the earlier non-OnStar cars, it was located next to the mirror housing. On the OnStar cars, it was incorporated into the housing.