I have a factory Cummins/Jacobs brand exhaust brake in the box I got off eBay from a RAM dealer on the East coast. I've always meant to install it, but never really felt a need for it. I have a 2006 4x4 crew cab short bed with a Lance camper on it. Weight over 12,000 pounds. But I have a manual transmission and I find downshifting works just fine to control my rate of decent while driving though out the West. I think I paid near $500 for it. Might want to check eBay or dealerships that might still have them in their inventory.
Maybe some day I can talk Reddog into motivating me to install it. Ha!
****EDIT****
Just checked on eBay and this is the same kit I have. This guy is way over priced I'd think, but the kits are still out there.
https://www.ebay.com/itm/OEM-Jacobs-Jake-exhaust-brake-Dodge-Ram-2003-2006-Cummins-ISB-5-9-engine-NIB/112882535646?hash=item1a4852ccde:g:G3AAAOSwo3tasd6L&vxp=mtr
