Did you install a new stereo? If so, you may connected the wrong blue wire to the antenna wire. If not, the power antenna works off of 3 wires: constant, ground and switched power. If you still have the OEM setup, the antenna could be going bad. Aftermarket replacements average around $60.
I don't know if this will help. I see that you have a C5. But look at my posting "Removing and replacing antenna mast." (C4 Tech) Also look at photos in my gallery. Detailed photos. My posting & photos may give you some idea. Any questions, feel free to contact me.
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