kia 2004 optima stock failure?

Hello, i have a question about a factory stereo in a 2004 kia optima. A friend came to me with her back speakers shut off, while disassembling her trunk to reach the amp the speakers came back on. i was not able to locate any short or recreate the problem. her front speakers were blown out so we went to BB and got some alpines. i installed these for her by clipping the stock adapter off and soldering/heat shrinking the factory to after market wires and then mounting the speakers. (i installed in normal phase). 4 days later her entire system failed and upon testing i found an under-volt from the head unit to the amp in the truck. 4.6V instead of 12V. she ended up taking this car to kia and they claim my soldering job was done wrong and this lead to the failure. I believe they are talking BS and intent to confront them on this but before i i want some feed back here.

but before i do* has anyone heard of anything like this? is there any possible way i am unable to think of that this took place? i primarily work on older cars but have installed 30 or so speakers systems including my own.