what is behind rear cargo bay

[glenn]

we are making the bus run on wvo need to run lines to engine . and bring coolant line to the second bay. and wires for the valves in the engine compartment to driver control panel

wvo is waste veggie oil 

[NJT5047]

On the MCI pictured in your avatar, if I read your question correctly, ony the drive axle, air tank, air valves, and suspension components are directly behind most of the wall.  The upper outer inner panel goes into the wheel well/inner fender area, and the center tunnel is an opening to the engine room.   You could perhaps run all of your plumbing thru the tunnel, or use the heater circuits?   
You may find a small tunnel in the upper street side rear bay wall.  Some MC9s had an air starter supply line running through a 4"X4" (or thereabouts) tunnel.  Exits just next to the upper inside corner of the rear junction box. 
JR