The old radio will have to be extracted from the dashboard using the appropriate Extractor Keys .
Then either purchase a replacement unit of the same size with the same connections at rear, or obtain the relevant fitting kit that will connect your new unit to your car`s existing wiring or plug/socket system.
Best to seek advice from your local Car Audio dealer as this can become awkward trying to find the correct fitting kit to connect the two together.(these kits can be expensive)
(If in the UK try Halfords )
RegWorts, August 2008