I now have the behind the seat Speakers in the corners

[Pontiac1976]

Well do you have the antenna in the windshield? If the if the little windshield ant adapter in place on the windshield, then you will only have to plug in the single black cable for the antenna looks like an RCA plug. The female to the windshield adapter and male to the back of the radio it about 3' extension cable. if you do not have one or the other you can find them at LMC or Classic parts of America.
you can remove the upper part of the dash to install the Adapter 1-1/2"-2" a snap button side to the windshield male RCA to the ant extension cable. Take out the radio let take the face bezel off route ant wires throw the dash reinstall the face bezel radio opening plug-in ant pop radio back in.