If we knew more about the switch, we could answer your question. Is it a door operated plunger? a toggle? a rocker”, built in to the light? Can you post a picture?
Also, I have a stupid question. Where is the “dome” light in your MGTD?
There are no more MGTD kit car manufacturers or “dealers” left. However, you may try MG Magic in Miami, Florida. They have an inventory of MGTD FiberFab and MIGI parts and may have what you need.