The phrase "third passenger" refers to someone who is not directly involved in a situation but is affected by it. They are like an extra person watching what is happening. Full definition
Rear Spoiler, MP3 Player, Power Third Passenger Door, Privacy Glass, Remote Trunk Release. (usedcarsgroup.com)
Rear Spoiler, MP3 Player, Power Third Passenger Door, Keyless Entry, Privacy Glass. (edmunds.com)
From the side or the rear, it seems larger than it really is, something you discover when you climb into the back seat and find adequate space for two — and a large transmission tunnel where the intended third passenger's feet are supposed to go. (articles.orlandosentinel.com)