Rear defrost wont turn off on 99 bonniville - Bonneville Pontiac Cars & Trucks

Has the rear glass been replaced? The wires to the glass are notorious for breaking near the upper most point of the hatch where the hatch opens and the harness flexes between the body and the hatch every time you open it.

on the rear seat

Chech the ignition timing is ok and is firing on the correct cylinder at the right time,

CHECK THE MAIN IGNITION FUSE IT THE ONE NEAR OR ON THE POSITIVE BATTERY CABLE

sounds like the defroster water drain plug is pluged, its under the car, once cleaned out it will release the sticky water and the defroster depends on the air con to remove moisture from the caben and defrost the windshied. Once you find the little rubber drain and clean with finger or compressed air you should be ok

IF THE BLOWER MOTOR ONLY WORKS PN HIGH THEN YOU NEED TO REPLACE THE BLOWER MOTOR RESISTOR LOCATED ON THE SAME HEATER BOX AS THE BLOWER MOTOR

Sounds like your ignition is failing. I would reccommend a mechanics opinion, however, those are very common symptoms of ignition failure.

u dont have a timing belt on 99 bonne and only a couple gm cars do, u have a chain

what are you calling a starter belt?
ther is no such thing!!!!

It is possible that the extra demand placed upon the charging system finally blew the alternator.
You need to have the charging system checked,,or check the output yourself with a volt meter (should be around 14-15 volts) test must be done with the car running.
If the voltage reads up to 14 or 15 volts then it would indicated a short in the dash control circuit.