I believe 9 out of 10 times it's the voltage regulator. The two spring loaded contacts on the regulator eventually wears down. You could be driving around with everything running off the battery without it being recharged sufficiently. The voltage regulator is mounted on the opposite of the pulley side with two flathead screws. (There are no wires.) There are some with diodes, some without. Have to remove it to check, but it's a Bosche part. You may need a very short screwdriver if you want to get to it from the top side. Roll up your left arm sleeve up to your shoulder first...