Summary

This is a mechanical toy. You push the car along and the heads bob up and down. I used my head (scanned) and a clay head (scanned). My head is the one on the right.
http://youtu.be/NLXSifecxOI

Instructions

Push the front wheel (with axle) through the body. Snap a wheel on the other side.
Push the rear wheel (with axle and nub) through the rear part of the body. Snap a wheel on the other side.
Paint the seesaw with heads
Slide the seesaw with heads into the top of the base.
Push the connector onto the seesaw.
Push the washer onto the back side axle of the seesaw with heads.
To secure the pieces that snap together, I touch them with a hot soldering iron (melts the PLA together). Gel type superglue will usually work also.

Pushcar