OK...I'm getting myself together after lol to Micheal's advice on an improperly installed dash...what a laugh. Anyway I seriously think it could be the surfaces as mentioned earlier, but from your description, the 'crackling plastic' to me is the climate control housing. This is a white plastic housing of the Evaporator that takes up most of the dash. It could be that part of something broke and is being blown around the housing or some device (if you have no pollen filter) went through the air intake. If its the latter, beware, this leads to premature evap failure- a monster to repair. Given your options, I think you should just wait else you will have to remove the entire dash to 'locate' this intermittent problem- a full day in itself. Just look at the other posts about AC repair to digest what it takes. Try to limit the fan speed to avoid all the debris blowing inside the housing or your plastic shrapnel may grind into the blower and cause failure then you are stuck in the middle of Cali with no AC and a bunch of pissed passengers.

I have had this problem. It turned out to be the windshield. It was wrongly installed.

Peer into your dash area through the windshield...you should not be able to see wires or plastic tabs or anything other than the dash. If you can kinda see four plastic tabs, and some wires, the dash support is broken. Often times, through several dash pulling required repairs, the four tabs that secure the "bones" of the dash break. I've used washers and hobby clay that air hardens into hard plastic to repair this. Someday someone somewhere will actually build a kit to repair this and make money, but it won't be me. I'll just stick with my Lowes washers and hobby clay.

Since I've always repaired these when the dash assembly was out of the car for another reason, I'm not sure if you can repair this without pulling everything apart. You might be able to just pull the dash pad, and the cowl (under the hood, you'll have to pull the wipers) to repair this.

Other than that...you could have some confused mixer doors for the climate control...they are just about everywhere under the dash...but it would happen any time you had the heater or AC on, not just on rough roads, and the climate control would not function properly.

Keep us posted...

Chris