[drover]

Assuming a 5 speed box then, its inside the box, if you google the solenoid adding a W5A580 tranny or NAG1 tranny you should get a hit.
If its been awhile since its had a service this could be the cause of problem, while they have a stupid 100K service interval changing at 40K intervals is far better option, these boxes certainly don;t like cruddy oil.... The 100K service regime is really only to lower the quoted service cost same with all brands.......

Some say a clearing of codes by battery disconnect will set things right but only for a short time, the service may require a good flush and clean out, ..... Use a Tranny shop for a professional job though, as they will be able to clean the box properly and check the solenoid operation , one that is familiar with Mercs is a good choice, a dealer will just stuff you around.