2004 Sequoia help

I am trying to put a trailer hitch on my Sequoia. I bought the hitch no problem but I cannot get the wiring harness. I called my Toyota dealer and they said they cannot order it and the only way to get the harness is to have it installed at the factory. I call all the hitch companies and they say I need to get a special wiring harness from Toyota. This is insane. I am on the wild goose chase. My Sequoia is already wired with the electrical connections under the truck by the spare. The only piece I need is an extension cable that plugs into the existing connection. I could just direct wire the connections to the existing plug but I think that defeats the purpose of Toyota installing a preinstalled electrical connection. Any suggestions?