Re: Malfunction Indicator light
A code is stored in the computer when the "check engine" light is on. In order to read this code, it is necessary to connect the car to a suitable computer. The dealer can do this, or you can find someone in your area who has a VAG-COM or VWTool software. Check the maintenance archives on these forums, there's an (out of date!) list in there.
Unless the car is giving other symptoms, there is no way to even hazard a guess of what's wrong without knowing the stored code.
And yes, it can indeed be caused by a faulty brake light switch or a bad glow plug.
'96 Passat TDI mit UPsolute