You're right, and as I suspected, I wouldn't have known the answer. The only backup camera I've ever worked on was on a huge motor home, and that one worked properly for many years, so we knew it was wired correctly and the problem was caused by something broken.
I have a long background in tv repair and because of that, my brain works best by seeing the wiring diagram, not by reading instructions. I might have figured out the need for the relay but then I would second-guess myself into doubting the need to re-engineer the product. If the camera was designed to work in your vehicle, the manufacturer should have tested it in one and included the necessary modification without making you search for the solution yourself.
I take issue with a couple of things your mechanic said. First of all, mechanics are real good at discussing car problems with other mechanics. They are typically not good at discussing car problems with car owners. To say there was "a signal problem" doesn't really say much, but to say "there's nothing they can do" is inappropriate. Where does that leave you? You paid to have a working backup camera installed, not a useless box. If they followed the directions correctly, and it doesn't work properly, the correct response would have been that they "don't know what to do". Many mechanics have a hard time admitting that.
At any rate, I'm happy to hear it's working. You still should ask for a refund because you did not get what you asked for. In the past I have gotten requests from the site owners to look at some questions that no one answered, to see if I could help. In those cases, I don't know, (or care), if the person made a donation. My third, and final career was as an automotive instructor, and I love it when people want to know more about their cars. I'm happy to help whenever possible.
You should be aware too that I don't reply if I only know part of the answer because once anyone posts a reply, your question goes off the list and no one else is going to see it or have a chance to reply. There have been cases where I knew the exact cause and fix of an uncommon problem but someone else answered first with some useless, generic reply. By the time I was asked to get involved, there were days worth of dozens of replies back and forth. Some of us are so eager to help, but then find out we can't see the diagnosis through to the conclusion. On other sites a lot of people can keep on posting and all the replies just get added to the list. It doesn't work that way here. Instead, you're having a conversation with just one other person. Most of us try to answer only those questions we feel confident we can find a solution, even if it takes days or weeks.
I don't wish car problems on you, but I hope you'll come back here in the future if you do. The stuff I mentioned in my first reply doesn't apply. You posted plenty of information. I wish more people put in that much effort.
Saturday, January 19th, 2013 AT 7:38 AM