1999 Mercury Marquis Intermittent Battery Light

The problem started when I replaced factory installed radio with factory built Cd player a year & half ago. The battery light started coming on intermittently. I had the alternator checked & it checked bad. I replaced and still had same problem. Eventually, the battery had to be replaced. I am on my third alternator but it does not fix the problem. I reinstalled the radio and the problem continued. To top it off, I can tap the battery light when it comes on and it goes out. I can drive for miles and it won't come on. Other days, it comes on, goes out, back on, etc.
Last symptom - the battery is reading over 14 volts most of the time even when the light comes on.

Any help would be appreciated.