Short circuit on radio power line!!!!?

it's my car,
http://farm8.staticflickr.com/7377/10165241196_7217b5f16e_h.jpg

a 78 Plymouth Volare. i tried to install a radio to replace the broken old one, but the power line is short circuit ( i didnt realize the problem at first ) when it's attached, and i used light wire instead, and it's short circuit again. the fuse of light is burnt which is easy to replace, however the power wire is definitely short circuit and fuse was replaced by previous owner by a iron rod. it could get burned somewhere else in the dashboard or so, and i have to take it to a shop to get it fixed....

how much could a repair be on this kind of old model? i already get the broken original radio in shop for $20 but i just wanted a cassette, it doesnt work though. it looks complex to get the power wire replaced i think......... how much and how long would it be?