Might need a little bit more info here but testing a simple whip antenna is pretty simple. I'll assume you have a shielded cable with a plug on the end. You'll need a volt/ohm meter and a jumper wire.
First check to see if the innner and outer contacts are shorted by holding the tester leads on the inner and outer pin and contact on the side of the plug. Anything other than infinity ohms isn't good.The cable is most likely bad. Next check to see if you have good connection between the outer contact and the body. 0 to1 ohms is OK. Now check to see that you have continuity between the center pin and the antenna. This is were the jumper wire comes in handy as the leads are probably too short reach. 0 to 3 ohms is OK. Lastly check to see if there is continuity between the center pin and the body. Any contact is bad.
Common adapter connecters can be found at Radio Shack and Auto parts stores. As far as 12 volts is concerned I have a 12 volt
circuit in the cabinet over the front couch.
Hopefully I guessed correctly on you're lead-in wire, if not post some more info.