Power supply for testing

Hi,
I want to check brake light circuit as the light has stopped working. I read an article on the MB website that recommended using a battery charger with red wire connected to yellow wire that connects to regulator and black wire from charger connected to earth example engine casing to provide a remote supply instead of having to start the engine. The battery charger I have doesn't provide any power although it works ok on my car battery. Not sure if its the type of battery charger which is an automatic RAC type that doesn't become live until its connected to the car battery. Any one tried this method for a remote supply? Scooter is LI150 special. Thanks in advance for any help.
I want to check brake light circuit as the light has stopped working. I read an article on the MB website that recommended using a battery charger with red wire connected to yellow wire that connects to regulator and black wire from charger connected to earth example engine casing to provide a remote supply instead of having to start the engine. The battery charger I have doesn't provide any power although it works ok on my car battery. Not sure if its the type of battery charger which is an automatic RAC type that doesn't become live until its connected to the car battery. Any one tried this method for a remote supply? Scooter is LI150 special. Thanks in advance for any help.