If you cannot get the immobiliser link plug from a dealer you can make your own from parts detailed on the Alternative Parts List
https://www.mtwc.co.uk/the-5-speeder/ on page 7. The plug diagram is also available in the Morgan Parts Book on page 66. All this information is freely available from the Morgan Three Wheeler Club and there are a large number of M3W articles available to members, including one about the immobiliser.
If your battery is the original Banner one it could well need replacing as they are very marginal for the M3W. There are a number of suggestions in the Alternative Parts List for replacement batteries that fit the space and have better performance.
