aux batt in, new probs

Ahhh.... this warms the c-ockles of my heart, and vindicates my obsession to DETER the use of isolators! You are ruining the front battery! You are likely getting a charge sense from only the back battery, and WAY overcharging the front one... The isolator isn't telling the alternator to stop outputting when both batteries are charged, only when the back one is done. Since the audio system is powered off the back battery, it never gets a full charge, (the system is pulling more than the alternator can put into it) thus never telling the alternator to turn off or slow down. Replace both batteries now, and wire them in parallel, WITHOUT the isolator. To do it correctly and safely, both batteries MUST be the same type and age, this is why I am telling you to replace them both. This is also an indication that your alternator is too small for your system. Do the Big Three, and upgrade the ULTIMATE POWER SOURCE IN THE CAR - your alternator. The batteries don't magically make power, my friend! They are there primarily to start the car and secondarily to filter the electrical power produced by the alternator, NOT run all your accessories!It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."