Hi Andy and welcome to mx5life.com
I have had two Mk4 (ND) MX-5s like yours and the second one, which I bought used at only 3 years old, had the same problem as yours with a rusted and frayed cable which jammed the window open.
I have since learned that it is a very common problem and the only solution is a complete window regulator assembly from Mazda which includes a new cable and pulleys. The cost was around £50 three years ago and a fairly easy fix once the door card has been removed.
I suggest you spray lots of liquid grease into the cable before reassembly to help it last a bit longer in future. Give it a really good soaking to allow the liquid grease to penetrate the cable strands. There are several makes of aerosol liquid greases you can buy and they are very effective.
Good luck,
Kev.
