Well I have not installed one, but If they are like most you can take 12v from just about anywhere.  There is most likely three wires on the unit.  One for power, One for switched power and one for ground and the antenna wire itself.  The kicker is if the radio has a power antenna lead.  This works as a relay to tell the antenna to go up and down when you turn the radio on.  If you don't have this lead then you would have to install a manual switch otherwise it would just stay up all the time.

If you have never replaced the antenna it is a real bear.  I think I had to take about have my car appart to get it in.  I'm sticking with my solid mast unit.

A factory installed power antenna has a relay mounted inside the rear of the console...it controls the antenna's operation. The one on Ebay may or may not be like that, but that's not necessarily bad.

A universal type power antenna could be hooked up direct to the fuse box or even the battery since it's not far from the installation, but install an appropriate fuse in-line. One wire would be power, one wire would connect to the radio power line to send the signal when the radio is off or turned on, or it could be connected to a manual switch in the cockpit. A ground wire would have to be run to the chassis. If you have or intend to install a modern stereo/CD player, virtually all have a wire specifically for a power antenna.

To install the assembly, it would be easier to temporarily remove the left muffler. The factory generally installs a heat shield around the antenna motor due to it's closeness to the exhaust.

If you decide to do it, it's not a tough job, but like most things on a Vette, it can be time-consuming. Both my '69 and my '78 have fixed antennas, and I'm OK with that...simplicity and less to go wrong.

'74 'Vettes never had a power ant. from the fact. There is more room on it than the other years to get one in & out, but.... snaking the ant. lead thru the rear bulkhead can be a real PITA.
You would have to get power from the radio, or a fused location that is hot with the key on. Make sure you have some kind of 'ground plane', like the one that should be on the ant. now, or it won't work worth a hoot anyway. Adams' Apple38524.817349537