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Actually, sometimes it all depends on the company. I've had company's that only cover $1000 in aftermarket parts... Wheels/oversize tires/bumpers, lifts n all that stuff. So the coverage wasn't so good.
My particular insurance policy covers the damages of aftermarket accessories up to the cost of a factory replacement. Optional coverages insures the aftermarket parts for their full value.
Usually they will only cover OEM replacements unless you specifically list the upgraded part. If you are insuring a vehicle for a certain amount and then dump thousands of dollars in extra upgrades without paying the increased cost to cover those parts then you will find yourself SOL.
A buddy of mine had some Dodge Dakota special super duper sport model truck and he tricked it out with some special hood that cost him something along the lines of two grand. I thought he was nuts but hey, that's his choice. Someone backed into him and the insurance would only cover the cost to replace the original hood. He sniveled about it but he never increased the value of the truck to cover the cost of the hood. So he was out of luck.