I have a 2000 grand prix and evry once in a while

Yes, the switch is the next thing to check.

https://www.google.com/search?q=2008+pontiac+grand+prix+rear+defogger+not+working

https://fuse-box.info/pontiac/pontiac-grand-prix-2004-2008-fuses-and-relays

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The first step is to check all the fuses also check in the engine compartment for another fuse box. the problem may just be a fuse or may be the control.

it sounds like the blower risitor. it is located behind the glove box next to the blower motor.

Heated rear windows have a high voltage load, quickest and easy resolve would be to install an inline suppressor to where the aerial plugs into the back of the stereo, available from all car audio store, it will improve your reception also.

you need to replace the resistor
it's on the right side under the dash

Believe it or not, but it was a bad ignition switch. I the switch apart and found about 3 different and one in particular that were burned pretty bad. Must have been the circuit that controls the blower and DRL? Found the solution on another blog. I put a new switch on in about 1.5 hours. I does take a couple special tools, very small female torx sockets. Other than that pretty easy.

check your wiring harness where it hugs the engine on the right front side, because of vibration this area is prone to broken and shorted wires

No solution found as of yet.

Check the ground, the resistors are accully resisnting the Negitive Current.