a safe bet for cleaning if you can gain access to the guts of the cart is a high polymer eraser.

old computer cleaning trick, applies here though.
it removes contact corrosion from a bi-metal connection as well as much other things.

also it leaves a bit behind so its not exposed to air but doesent hinder connections, plus, its rubber.

a polymerized rubber but still rubber and not conductive should something go astray.
if i remember i'll post some half and half pics of a dirty cart