Does anyone know if it's easy to open up one of those little cars and
get at the relays that control the car, to control other things than
the car? (ie hack into a lego mindstorms set, turn on a TV, etc.)


There aren't any relays these days. Stuff like motors and electromagnets (used
to steer the little ones) are run directly from transistors or chip outputs.

The very tiny HO scale cars have only 3 outputs, all fairly low current and
limited to about 3V. I'm not sure what's in the larger vehicles today, haven't
fooled with one lately. RS used to show the circuit boards for the cars and
transmitters as separate hobby items (with instructions) in their catalog, but
may no longer do so. Why not check out someone like LINX on the Internet, who
make transmitter and receiver modules. Their application notes show many
interface examples.

Doug Moore