Sunrrof \ Windows stop working when engine running

My sunroof and electric windows stop working as soon as the engine is started.

There is 12v at the window switch when the ignition is on, however there is only about 8v when the engine is running. At fuse number 3 there is approx 12v when the engine is off and about 14v with the engine running.

When the engine is off the windows work perfectly so this does not appear to be switch related.

Anyone have any ideas? What would cause the voltage to drop at the switch but only while the engine is running ?

I have cleaned up the earth that is used for the windows.

It makes no differnce.

Thanks anyway.

I would suspect an "earth" somewhere where one shouldn't be...but why only when the ignition is "on"? Haven't a clue, but you might check your ignition switch wires for a start...