Hello, the "electric system" of my 72 twin cam likes to joke sometimes...like the right indicator sometimes works,sometimes no. So i'd like to do something, i was thinking to start from the earthing (ground) points, but i don't have idea where they are. Suggestions? 
As BDA says, this sounds very much like an earthing problem. The light units normally have an individual ground wire (black) which goes from the mounting bolts on the base of each unit. Often the same earth wire will be linked to several lamps and then, to make matters worse, perhaps a bullet connector at some point in the loom before it goes to the chassis as a main earth conductor.
Trouble shooting is simple but slow. For most checks like this I have a long length of wire with crocodile clips at either end which makes it simple to provide a certain earth link. If that makes the light work, then job done. For something which is intermittent it's less certain so as a first shot I would remove the lamp base, clean it up and re-make the earth connection. 9 times out of 10 that will solve the problem.
For a long term solution it's fairly simple to buy the connectors and make up new earth wires with soldered connections. I used a heavier gauge wire and it's basically fit and forget for my lifetime !
Brian
edit to add - for things like the front indicators and anything else in that vicinity I often find it easier to remove the front grille and work through the opening rather than struggle round the bonnet lid. In fact my grille is held on with two small hinges at the bottom & cable ties at the top so it can lie flat when needed. Heresy I know, but it's so much easier.....
