Mustang Undervoltage Issue

Whoa, that sounds like a real headache! Undervoltage in a Mustang can be tricky. First thought: have you checked the battery's resting voltage with a multimeter? Should be around 12.6V when the engine is off. Also, while the engine is running, what voltage are you seeing? Anything below 13.5V or above 14.5V could point towards an alternator issue. A parasitic draw could also be the culprit, slowly draining your battery when the car is off. Checking those numbers would be a stellar first step!

Thanks! Could you elaborate more on how the alternator would affect the system voltage? I have limited knowledge and want to make sure I've covered my bases before taking it in.

The alternator recharges the battery while your engine is running and provides power to all the electrical components. If it's failing, it might not be putting out enough voltage, hence the low voltage and symptoms. Also, it could be overcharging, which is equally bad! Given the electrical issues, it points towards a charging system or battery issue. Honestly, with those symptoms, and without proper diagnostic tools, it may be best to take it to a trusted mechanic. They can do a proper load test on the battery and check the alternator's output. I had nearly identical symptoms which turned out to be the alternator output being too low.

I appreciate the information and advice to consult a professional. I took it to my local technician and they identified a heavily discharged and defective battery. The total repair cost was 290€. Thanks for the help!