The companies authorized by Alpine to sell parts are -
www.pacparts.com
and
www.hermanelectronics.com
Usually i use pacparts to find the part numbers, since their search feature is much more efficient, then i purchase from hermanelectronics, cos they're in Florida, while pacparts are on the west coast.
in this case, the part number you're looking for is 88T55261W01 and the price is $25.85 + Shipping.
All you have to do now is drop it in a sky-box and u should have it in a few days!
