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Its for installing an auxiliary audio device, like a CD changer or a satellite radio. You have to remove your radio to attach a device to it. There was no auxiliary input in the 2003 Toyota matrix and was not optional the first aux was for the 2006 matrix and up
The 2006 Toyota Matrix runs on regular unleaded.
The 2013 Toyota Matrix runs on regular unleaded.
The 2011 Toyota Matrix runs on regular unleaded.
The 2009 Toyota Matrix runs on regular unleaded.
The 2004 Toyota Matrix runs on regular unleaded.
The 2008 Toyota Matrix runs on regular unleaded.
The 2007 Toyota Matrix runs on regular unleaded.
The 2005 Toyota Matrix runs on regular unleaded.
The 2010 Toyota Matrix runs on regular unleaded.
The 2012 Toyota Matrix runs on regular unleaded.
The 2003 Toyota Matrix runs on regular unleaded.