dump trailer hookup

[1985 kenworth]

Hello everyone i have a question .i bought a dump trailer with a screw drive motor and i want to hook it up to one om my trucks that has the mfu in it and it has the dx7 radio.i am currently only using 4 channels on the radio but was wondering if anybody could help out with some diagrams or advice how to set it up on a channel.i was told a 3 position channel would work for this.currently i have the dump motor plugged into a separate esc i bought for the trailer and it has its own bettery pack a nimh for this esc.i have an extension plug from this ecs to run under the trailer to be plugged into a channel on the truck receiver so i can lift and lower the box by a channel on the radio,also the esc for the trailer when i turn it on is flashing red ?