where is the brain box at on a 1999 model

It on the firewall underhood, passenger side .click image to size up

Opps that was diesel but its almost same

What's the problem? What leads you to believe the ECU aka PCM is bad?

Dodge calls it PCM but I don't like to use that, can be confused with PCM --Power Control Module for electronic throttle, aka Drive-By-Wire and prefer ECU or just computer or brain-box, like you used I knew exaactly what you meant.