Which year subaru has a Viscous Coupler?

You can NOT UNcouple the viscous link in manual transmission Subies, as they are locked and fully mechanical. Don't look for a fuse to short out the transfer case ala the 4EAT automatic. These modular viscous couplings will fail with time, losing the ability to liquefy the viscous fluid as temp changes, resulting in "crabbing" or difficulty parking, acting like a locked-up differential. This can be hazardous in tight radius slippery conditions. It's about a $600 part that requires a couple hours to swap out if needed. But NO switches, relays, nor solenoids control it. Sorry.