If I remember correctly, you should have one wire from the raido for the power antannae and three from the stereo harness to the power antannae relay then onto the antannae.  I am getting some work done on the Vette now and my manual is at the shop.  Good luck, I had to drop my entire exhaust to replace my antennae, FYI.

The top plug in to the radio is for the power, ground and dial light connections and antenna out wire. The pther 2 plugins are for the speakers. In that top plugin the top wire(yellow)is for the power. The wire right next to it(yellow or possibly orange)is FROM the radio and goes to the power antenna to trigger it up and down. The rest of the antenna wires are seperate from the radio wires. Your original etr radio will have an additional wire for the memory that you won't use with the 78 radio.  Mike

One more thing you should check is for power to the relay then from the relay.  If your relay is bad, you should change it out before you tackle the antennae.  I had no fun changing out the antennae.  The relays are hard to find also.  I have one that works if you need one. Rick