Here's the question. Where does this wire tie into the system?
If the amp is drawing from the starter relay or the battery, then that can be a problem. The ammeter won't read charge/discharge correctly and if the audio amp draws high load, then the what should be just a charging circuit can get overworked.
If the amplifier is pulling direct from the alternator or the fuse box and the engine is running, the ammeter & wires to it are uneffected. If the amp is run with the engine off, then the ammeter circuit can get in trouble and should be bypassed.
See this: Understanding Charging Systems with Ammeter
(and if the whole Volt - Amps thing needs refreshing, this post)
