Battery charge light on the dash? Or perhaps you're having to constantly recharge. Chances are it's the alternator at fault. Don't risk 2nd hand units when you can get brand new items at this price! Quality aftermarket alternators for a direct fit.
Unlike the Mk1, all Mk2 MX-5's used the same alternator from 1998 until 2000 when the Mk2.5 was introduced, which again, used a different design to the Mk2. Please ensure you order the correct alternator for your model of MX-5.