battery drain?

i have a 91 lx 4cyl. recently replaced the alternator. my battery reads a good level when I'm driving, but as soon as i stop and idle my engine it drops. it goes down to about 9 volts when the engine fan turns on (this is with heat, lights, and music going). the battery goes so low that i can see my lights dim. i don't know if my car is idling too low (it now reads 700 rpms), or if maybe the voltage regulator is not built into the alternator in mustangs? any thing will help.

Something like that used to happen to my friends car. He also had music and all those other accessories and he found out all he had to do was get a battery with more cranking amps. Afterwards he didn't have any problems with it. Just thought it may help.