Google " Invisibrake " to see if that's the aux toad braking system your looking at .
Should have cable under the carpet , a pulley on the firewall and a connection, for the cable ,on the brake pedal arm about 4" above the pedal .
EDIT: Back in time to add to this post .
Could be part of the " USGear Unified tow brake" system , if what's under the seat looks like this.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.