Have you seen this? SOLD - ND (1.5 or 2.0 litre) i-eLoop DC/DC Converter DJ4V-67Y6Y-Revision C For Sale - Buy and Sell [Replies by PM System Only] / MX-5 & Roadster Parts For Sale - MX-5 Owners Club Forum It shows the part you might need.
Also this Battery charging malfunction - Technical Area / Electrics & ICE - MX-5 Owners Club Forum which has the most information on this subject.
