Dude, it's a yellow box inside and underneath the driver side rear light panel. To get access to it, you need to remove the panel where you normally need to remove to change the light bulb. Then you should see the box just underneath it. There's a #8 or #10 bolt holding it on. ( not sure which one ) But you will need a socket wrench and an extension to remove it.
About testing it, not sure if you can. Just replace it if the light on the dash won't go away after trying to replace all the rear bulbs. that should solve the light issue.