Perhaps you are talking about the Interrupter Relay IR1000? A small grey potted elect box that is wired in-line usually right behind the switch?
On older Bennett controls there was a possibility of trying to put one side up and the other side down at the same time. This would cause the pump motor to turn in forward and reverse at the same time and would blow the fuse. The Interrupter Relay would cause nothing to happen should the controls be in conflict and not blow the fuse.
You can just take it out and not go up and down simultaneously, worse case you blow a fuse.
The other "black box" is the "Auto Tab Retractor." This unit retracts your tabs automatically when you shut off the engine. I can be identified by the 22 gauge orange wire going to an ignition circuit. It too can just be taken out of the system and it will work fine, just not have that feature.
That may not be your problem. There are several troubleshooting tutorials on the Bennett site www.bennetttrimtabs.com
A few things to look at
the fuse, inline 20a
all screws on he back of switch are there, clean and tight
the connectors are notorious after 30 years for having bad connections
ground from pump to ground is bad