Whenever I press the brakes my voltage drops from about 13-14 to about 9-10... why is this? Also, whenever i turn on my rear defroster, it will dip to this low range.... this has caused me a lot of problems, because with this low voltage, my blinkers blink slower, causing a lot of near misses.

 

It may be a fusible link @ the starter - there are three of them attached to the large lug on the starter. Check the voltage under load there & then @ some points down the line (@ fuse box) to determine where the voltage drop is occuring. The brake lights & turn signals are powered by the stop / hazard fuse, which is powered from one of the fusible link feeds.