rendevynz How often are Sarawak & Sabah plate in mainland? and how hard it is to bring the car from there to mainland?
As a local Malaysian, I frequently see East Malaysian plates (Sabah and Sarawak) ones on the mainland. Many Johor people use Sabah plates for its similarity with Singapore ones.
However, I'm not very sure about the process of bringing East Malaysian cars to the mainland. I only know that shipping costs can be high. People on the mainland (for example Johor) can actually register their vehicles there, using East Malaysian plates.
rendevynz How often are Sarawak & Sabah plate in mainland? and how hard it is to bring the car from there to mainland?
If I'm not mistaken since roughly 10 years ago, you do not have to register a car in the state you live in, so you can choose to have an east Malaysian plate in west Malaysia and vice versa. Therefore I think most cars that have these plates might have never even been in that state, the owner just chose to get a license plate from there.
Having travelled the country recently I would say there's plenty of east Malaysian plates in west Malaysia (not sure what it's like in east Malaysia as I haven't been there yet).