Failed earth causing indicator failure.

I have my second MOT this friday and as with any MOT I have a look at the basics such as lights etc. so I don't fail on something daft.  As it happens my cycle wing drivers side indicator wasn't working.  I ordered another from Caterham just in case (super fast delivery as always) but thought I would also check if power was getting to the indicator before replacing it.  Having taken the wheel off and looking under the cycle wing you could see some corrosion.  I took out the nut and bolt where the indicator was earthed and it was obvious there was no chance of a connection.  A quick clean with a file, re-connection and then waxoyl over the area and my old indicator is working again.