yeah, its a pain to put a cable in but it should only take a few hours, and cost less than £20!
what current does the amplifier draw? if the amp has anything bigger than a 10amp fuse i would run a power cable. As FWH says above, you could burn your car and possibly with somone in it!!!!!
a company called Celsus make a lot of line out converters i have this 1
Car Audio Direct - Celsus LC2 Adjustable Converter - Accessory
although i havent installed it yet, this is the sort of thing you will need.
and if you buy the power cable from the website at the same time you could save yourself loads of money!