http://www.antennax.com/Products/Description_ax05.htm
I bought an aftermarket power antenna here for my 1981 Corvette.
If the wire is still intact from radio to the back, you can just buy an
adapter for the old Corvette screw in type connector. Radio Shack etc...
The original Antenna wire for the 81 Corvette, was in 2 parts. A LONG section from Dash to the back driver side , ( Way back near back wall of the storage space.) Then from there to the Antenna itself...about 15 inchs.
If your LONG section is intact, just place an adapter on the end near antenna....then just run the antenna wire from the aftermarket unit up into the rear compartment and plug in....