As I've stated before, Disney did this for a reason. People were trying to avoid having to pay to park, so they parked at DTD, where parking is free, and then hopped on a bus going to a park. It got to be quite excessive, so they stopped it.
Most people head to DTD from their resorts, rather than from a park. But, if you are at MGM, you can walk over to BW and take the bus to DTD from there. If you're at Epcot, either BC or BW will do. At AK, then you'll have to get to AKL and take their DTD bus. MK obviously would have you take the bus from the CR.