At any rate, if you bought it used, the previous owner probably turned it off through the disable process through the EVIC. The procedure to turn it off is in the manual so I assume you repeat the procedure to turn it back on.

Disable BeltAlert (Seat belt Chimes):
NOTE: The following steps must occur within the first
60 seconds of the ignition switch being turned to the ON
or START position. DaimlerChrysler does not recommend
deactivating Beltalert.
1. Turn the ignition switch to the LOCK position and
buckle the driver’s seat belt.
2. Turn the ignition switch to the ON position and wait
for the Seat Belt Warning Light to turn off.
3. Within 60 seconds of turning the ignition switch to the
ON position, unbuckle and then re-buckle the driver’s
seat belt at least three times within 10 seconds, ending
with the seat belt buckled.
NOTE: Watch for the Seat Belt Warning Light to turn on
while unbuckling and off while re-buckling the seat belt.
It may be necessary to retract the seat belt partially each
time when unbuckling.
4. Turn the ignition switch to the LOCK position. A
single chime will sound to signify that you have successfully
completed the programming.