Yes, I do have proof, they told me that they did and I was there waiting for the vehicle. It wouldn’t start after the battery was out in and an experienced mechanic came to check it and diagnosed the problem. They said that the old battery was put in backwards (physically) and when they replaced it, they just hooked the cables back up (and I am presuming they never checked how the old battery was hooked up) but to the wrong terminals. It sounds like it was careless work not checking the cables/terminals correctly when disconnecting the old battery.
The old battery was tested by my regular mechanic and came up as “bad” with 48% charge. It had been parked with the lights on for 30 minutes and had to be jumped afterwards. It was probably about 5 years old. The alternator light had never come on before.
I also noticed this afternoon that it wasn’t starting smoothly but cranking a while before hand. I’m now beginning to wonder if other systems were affected such as the starter and the onboard computer which may not give the right mixture. The blinking rear door dash light could well be a faulty sensor caused by the incorrect wiring. Reading around on other sights it seems the clock/radio could be out because of “fried” wires.