If you use your slow cooker regularly, the inside will eventually get dirty. Things spill or get on the outside of the liner. No need to scrub it -- let some household chemicals do the work!
One Good Thing suggests putting a small bowl of ammonia in the cooker, then putting the lid on to let the fumes release the gunk. You don't need to plug it in -- just let it sit. This isn't for cleaning the liner; those are usually dishwasher safe. It's just for the inside of the actual cooker.
If you are sensitive to ammonia, or just don't have it around, try vinegar and bicarb soda.
A Simple Way to Clean the Inside of Your Slow Cooker [One Good Thing]