If it's like the trooper, there's a button switch on top of the brake pedal
that you can see from underneath. When you push down on the brake, it lets
the switch drop down and change the connection state. Just look for the
wire coming off of it.
You should be able to get a wiring harness that will hook in easily as well
as handle the conversion. The Trooper has a plug up under the rear driver's
side fender...makes life easy...hopefully the Rodeo is the same. If not,
you can still get a plain ole tail light converter at most any place that
deals with trailers, campers, etc. Even Wal-Mart has them. But, I'd see
if you can get a harness first - much easier.