I second Scott's suggestion. When I first got my 857, I also bought the LDG Z-100. This set-up was for my vehicle with the 857 body & Z-100 back near the battery. The 857 front panel controller is near the driver. Tuning was a pain because there is no remote control on the Z-100. Tuning required stopping the car and opening the hatch. Ugh.
The LDG YT-100 with the Yaesu cable derives its power from the 857. No separate battery or power supply. All I needed was a remote push button near the driver to trigger the YT-100 tuning. It works quite well in this configuration.
John AA6ZT