Simple as disconnecting the two hoses and hooking up a low pressure water source to the passenger/inlet port?

 

Oh, is backflushing accomplished by connection a low pressure water source to the driver/outlet side?

 

Yep disconnect the two hoses (I honestly cut them off to avoid hurting the solid heater core lines). You can just get 5/8 heater hose & cut it to length vs using the OEM $$ hose. Then flush it backwards first with a hose, then when it flows good and clean flush the opposite direction. In/out hose direction doesn’t matter as it’s just a loop.