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Message: <blockquote data-quote=".Mike" data-source="post: 444247" data-attributes="member: 22637">Nobody has offered a single suggestion that explains how a bad black box could cause the interaction you are experiencing between light dimmers and your trailer lights. An intermittent ground (in the black box, or elsewhere in your vehicle), or even ground potential, explains it all day long. So does interference, which a 2 minute web search reveals can be an issue with Sienna vans.5 minutes with a voltage tester, following the suggestion I made, will allow you to determine if you need to spend the extra $60. Randomly replacing components is guessing, not troubleshooting. You've already guessed once for $60. Do you really expect that another $60 guess will produce a different result?What is the harm in following a reasonable troubleshooting procedure? If you aren't measuring the voltage going in and out of the black box, you aren't troubleshooting. If you aren't securing the grounds and testing for resistance or ground potential, you aren't troubleshooting.</blockquote>