You can check the voltage on the battery with a multi-tester. Set it to the 20 scale - DC. Black probe goes on neg. terminal of battery; red probe goes on pos. terminal of battery. Fresh batteries read 12.60. There are MANY opinions as to what reading indicates the battery is getting tired. I replace mine when they get around 12.25. This test by no means addresses load, but does give good indication as to a batteries ability to accept charge.
You live in a large city. Check with local euro parts suppliers as I did. The $71 dealer Bosch brush can be had for around $30 in your area.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.