I need to know how to install an aftermarket HU in a 1989 Chevrolet Suburban 1500. I bought all the necessary stuff (harness, adaptor, antenna cable) but the harness does not match up, its too big and its labeled 86' and up GM. He said there is an external amplifier that comes stock and that what i need to wire it up to. i personally think he is blowing smoke bcuz when i took out the old one there was just a regular harness. Please help i have to have this done by friday. any help greatly appreciated. thx