Originally Posted by BlackWorksInc

Usually when it comes to add-on stuff like lights and such, Toyota (not the dealer, actual Toyota side) will ask to return the vehicle to stock configuration before proceeding with any further diagnosis on issues to eliminate the possibility of it being non-warrantable and/orb affecting the stock components. I made a post about mods and warranty on another thread recently.

Sometimes you'll have to pay out of pocket for diagnosis to have the dealer prove it isn't affecting a warrantable item. I will also add that in general, most technicians will rule out aftermarket stuff if it's easy, i.e. just disconnecting a piggy back connector or wire. But from a warranty perspective they don't get paid for diagnosis even if it wasn't modified, so the more time wasted on diagnosis is money out of their paycheck; so it kind of depends on how generous the technician/dealer is feeling. Most places are willing to work with you to an extent, but after a certain point you'll end up needing to pay for additional diagnosis time or return the vehicle to stock configuration yourself before they'll do any more "free diagnosing."

Depending on how you install the items (i.e. cut & solder, wire taps, or just piggy back connectors) they may or may not honor warranty on that harness/attached components and/or require you to pay for a new harness to honor warranty because you "damaged" the factory wiring.

It's one of many reasons I advocate for people to just set up a second fuse box with a dedicated power and ground to use for their aftermarket electronics where possible. Not only does it make diagnosis much easier but if you did have a warranty issue and they tried to give you crap you just pop the positive feed cable to the fuse block and say, "back to stock, so back to diagnosising you go!" Lol