Does anyone know of a bus service that connects Berchtesgaden with Garmisch-Partenkirchen or would I have to travel back to Munich by train and change there?(no car)
also has anyone travelled to Colditz castle from Leipzig?

Bercht. - Garmisch:

Because of the distance and the Alpine terrain, the train route via Munich is your only practical option.

I just came back from that area and I never figured out a connection. I think you will have to do the trains. Be sure to check out the Bayern Ticket, now 15 euros per person, which is good on any local trains in Bavaria after 9 AM on workdays and anytime on weekends.

The Bayern ticket is a good deal - but the restriction to local and regional trains means a slightly longer trip, so check your schedules closely if time is important. Also, the 15 Euro/person price is good if traveling alone, but if you have company, there is also a 22 Euro price on this pass which allows up to 4 others to travel with you.

Thanks Russ and Karen for your suggestions and insight. The bayern ticket sounds a viable alternative. I will check it out. Thanks again