Couch Potatoes (Human!Impala+TARDIS)

Impala became a human when the TARDIS did: The Doctor, Clara, and John Winchester in the 1980s ended up working on the same case, but barely interacted with each other throughout it.  Funnily enough, the TARDIS and the Impala always ended up parked right next to each other wherever the three of them went).
Eventually, something happened to the humans (and Timelord).  They were in danger and the TARDIS knew this and desperately wanted to help, but couldn't because it would never fit in the confined space the Doctor, Clara, and John Winchester were trapped in.
Having only a small moment of time to calculate her options, the TARDIS came to a conclusion: It needed to become human.
It took a great amounts of power, but the time machine transferred its energy into the nearest source of human life that could contain her power.  It was a long shot.  The body it found belonged the a young female comatose patient in a the hospital the TARDIS was parked outside of.  The TARDIS didn't fully understand why, but this woman's inhabited spirit was strong enough to contain the time machine's power.  It shouldn't have been possible, but it was.
TARDIS entered the body and broke out of the hospital as fast as she could (her motor skills were pesky things, how do humans STAND this?!).  She went to where the Doctor and everyone were being uncomfortably squished and broke them out.  Yey!  A confused Doctor thanked Tardis (who could remember how to talk yet, stupid tongue!).
What Tardis didn't know was that something was happening to the Impala during her transfer into a human.  The energy that linked the TARDIS to the comatose patient seeped in the earth and in the objects around the blue box.  Most inanimate things ended up floating off the ground and one fire hydrant grew spots, but the most radical change was to the Impala.  Using the memories and history behind Impala (reflecting on both the past and future of the vehicle), the car transformed into a man.

Thus began the long, exciting adventures of the TARDIS and the Impala!  Tardis discovered quickly that, when in an exact spot in time during a solar alignment, she could transfer herself into the comatose patient's body without exhausting her energy.  Whenever this transfer occurred, Impala too would become human, no matter where he would be in the world.