yes they can be retro fitted, and yes it is quite easy, basically you need to track one down off ebay, golf mk4 ones will fit and loads about on there, And i think you just need to run a power supply up there, off the iginition and thats your lot, Check ukmk4's site as there a re a coupl of guides, do a search on here,
there is a guy, foxy i think who has a site for his golf, he seems to have retro fitted virtually every option available,
Don't be put off its easy, if i find the link i'll post it.