Engine Low Idle?

I don't know what kind of tester you have but, if you have a voltmeter hook it up to the battery and observe the voltage. Start turning on accessories one at a time till everything is on, radio, heat wipers. And see what happens as you turn each thing on, at the end you should dtill get 13.8 volts-15 volts with the engine running. If it keeps sinking change the alternator before anything else!