Cummins ECU ; gets a signal from the brake light switch , to cancel the cruise control.
Nothing from the signal switch .
Only the RR signal out, and both brake lights , unless the chassis has a rear fuse box for trailer wiring , you might have two issues .
Front fuse box , could have been relocated by the coach builder ; out side under the hood or behind an access panel close to the described location.
The Freightliner wiring harness would have about 5' of wiring to allow coach builder to mount where space allowed.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.