BMW Repair: Electrical problems, visor mirror, drainage problem

Question
As far as flooding my car has never been in a flood however last week Pa were I live had a lot of rain and I noticed that my rear passenger floor was a little damp but that was the only area and I assumed that it is just a drainage problem.  When I jumped my friends battery I attached the the positive to my battery and then the negative and then attached them in the same order to his battery while both cars were turned off.  This problem didn't occur though until a few days after.  I also noticed that along with the problems listed on the previous question when I for example push the horn it does not make a noise however the air bag light will dim.  Also I opened the shade on the visor mirror and when the light came on there my radio shut off.  It seems as though there may be a power problem because when I push one button it dims another or shuts it off.  However the car runs perfectly and my head lights and major electical systems are operating properly.  I also noticed on this car that my tail lights burn out more often than on any other car that I have ever had.  The only other condition that may be of importance is that is has begun to get colder this week but I am not sure if that would help you determine what the problem may be.  Thanks for your help.

Answer
you may want to check the condition of the charging system. if you take a multimeter set on DC to volts. with the engine running, measure the voltage across the positive and negative battery terminals. it should be around 13.5 volts. if it is not, you may have a bad alternator. however, when you jump a battery, on a bmw, under the hood, there is a positive battery connection to use for jumpstarting. never hook both cables to the battery. only hook the positive to the battery and the negative to a metal part of the car.