I often get fed up with people asking how I clean the inside of vehicles chassis so I will impart a little knowledge.
You will need a steel wire rope of around 10mm diameter, or several smaller diameter steel cables and they must be longer than half the length of the chassis.
Short length of heat shrink tubing which fits over the steel cable.
Powerful drill which means a corded drill and not a cordless drill.
To make your chassis cleaner you do the following:
Take your steel cable and place about 2" (50mm) of heat shrink tubing about 9" (175mm) from the end of the cable and shrink, now unwind the cable from the exposed 9" and form into a bush which is a tight fit into your chassis rails, put the other end into the drill chuck and turn at a slow speed and slowly withdraw from the chassis rail, blow the chassis out with a compressor.
With the chassis cleaned you need to do the corners of the box section so bush the end again so its a tight fit and simply rod the chassis as you would a blocked drain, the amount of rodding depends on the amount of debris inside your chassis, this will give you half a chassis rail cleaned of muck and rust. Start from the other end of the chassis rail and repeat, this will give you a clean chassis rail. Repeat the process on the other chassis rail for a cleaned chassis inside.
If you cannot get a piece of steel wire rope around 10mm diameter then get several smaller diameter pieces and any Bowden cable such as bicycle or motor cycle brake cables, or van clutch cables will suffice. Cut them to length and place a piece of heat shrink over them about 9" from the end, then pieces of heat shrink every 6" along its length to hold them together.
This will clean out the chassis rails but you may find that an assistant wearing leather gloves is needed to hold the rope in the middle to stop it whipping.