Hello. My 2000 Bonneville back glass shattered the other morning after warming up. The light on the rear defog switch was still lit after it shattered. Could any one tell me if the 2000'* timer is built into the panel switch or could it possibly be in the relay switch? If it is in the dash unit switch can the switch alone be replaced or would it mean replacing the control unit holding other switches? I would like to replace the timer just in case it was the cause. I appreciate any replies. Thank you.

I don't have a FSM for your year. But check the maxifuse panel under the hood on the fire wall.


Then pull the black covers off.


In this picture the furthest right fuse is the rear defrost.


Also check the relays. If it'* staying on, the relay might be stuck. Your owners manual might tell you which relay it is.

there is not one of them in the 00+. IIRC its controlled by the Climate controll headunit

IIRC the timer is for 15 minutes if you are above a certain speed, if the speed is less then the timer is on for longer. Pressing the button on the climate controll unit again will deactivate the rear deforst

The system is quite a bit more complicated than just a timer module.

When you push the defrost button, the control head sends a class 2 data signal to the rear integration module (RIM). The RIM then engages the rear defrost circuit. Once this is done, the RIM sends another class 2 data signal back to the control head instructing it to illuminate the "On" indicator. I would have to do further research to determine whether the timer is part of the control head module or if it'* managed by the RIM.

Further to this, there is also a class 2 data signal sent to the driver door module (DDM). This is a gateway module for controlling all door functions. The DDM applies power to the left outside mirror element while at the same time sending a message on a simple bus (connecting all four door modules) to the passenger door module instructing it to turn on the passenger outside mirror element.

I'm assuming that you believe the rear window shattered because the rear defrost remained on. While I suppose this may be possible, I'm not really sure how likely this is. There are circumstances when it remains on constantly by design. If your speed remains above a preset level, it will remain on to ensure your mirrors remain clear. This really only applies if you have heated mirrors, but I don't believe the system actually differentiates whether you do or don't. I believe the reason these turn off is not due to "excessive heat", but because this system generates a heavy load on the electrical system when it'* active. At speed, your generator has increase output and it'* not as big a deal. At lower speeds it can antagonize the electrical system a little more.

After browsing through my documentation a little more, I have some additional information. The Instrument Panel Module (IPM) communicates with the RIM to control the on/off function of the rear defogger. I believe this is where the timer control originates. Now, if you have dual climate control (option CJ2), the HVAC control module actually sends the request to the IPM first and then the rest of the process is described as above.

If your speed remains above 48 km/hr (30 MPH), the system remains on constantly. Below this speed, the timer cycle is 10 minutes the first time you press the button, 5 minutes for subsequent cycles. This is reset every time you cycle the key.