A few months back somebody posted a link for the Morgan Trad Parts catalogue. I saved a pdf copy and it is very useful - has exploded diagrams of almost everything.
http://www.morgan-cars.com.cn/wp-content/uploads/2013/02/morgan-traditional-parts-book-2006.pdfHere's the side indicator - as I thought, it appears to just be a push fit into the wing, and should be removed from the outside, not the inside. I haven't needed to do this, but have removed the lenses which are just a push fit into the rubber. The rubber seems to have a groove which may be stopping you removing them easily, perhaps working some washing up liquid into it would free it up.
Nigel