2003 Mitsubishi Eclipse electrical problem

Original radio and clock in the dash doesnt work but now it started to turn on the clock
but after a few seconds it goes dead again, check all the fuses and they are good took the
car to a shop to install a new radio and theyvtold me it has a short somewere
because some of the cables in the radio didnt have constant electricity when they supose
to have what could it be the problem?