Factory Service Manuals (FSMs) are a great resource for diagnosing and isolating issues you have with your Miata. They can reduce the "shotgun method' of randomly replacing parts and pieces. Often only basic tools and practices are needed.
Fortunately, someone has put 18 years worth (!!!) of Miata FSMs online in PDF format. This is a GREAT resource.
http://www.mellens.net/mazda/index.html
Example...the A/C on my wife's 10ae wasn't operating correctly, so rather than leave someone else to diagnose and repair it, I followed the FSM procedure for that system on her car and isolated the problem in about 30 minutes using a cheap multi meter and a piece of jumper wire. I was able to make repairs and have the A/C repaired the next day. I had the system recharged professionally, but because I knew exactly what was wrong, I was able to tell whether the shop that did the work was honest or not.