Cat 777D Mining Truck

[JensR]

I cannot really comment on the wiring issue, as I don't know how it looks inside.

However, I do believe that two "half channels" with two servo positions each is all you need:
Channel 3: light on / off
Channel 4: dump bed up / down until limit switch reached.

Anyway, here is someone selling an adapter to get proper three-position control:
http://wantasummit.com/default.aspx
http://www.rccrawler.com/forum/showthread.php?t=307196