My insurance policy with Norwich Union Direct has the following wording...
The cover also applies yo your car's accessories and spare parts while they are in or on your car or in your private garage. We will also pay for loss or damage to, your car's audio equipment or satellite navigation system (up to £500, unless it was fitted by the manufacturer).
When I asked some time ago, before I took out my first policy with them, I was told the car and all it's options would be covered, although I may need to provide a copy of the original purchase invoice showing what options were fitted, particularly in the case of a full replacement vehicle if it was lost, stolen or the cost of repairing any damage is more than 60% of the UK list price (including car tax and VAT)