Yes, corrosion of the connectors is a very real and very common situation on all trailer brakes. When and if you get new Axels, Dexter at least makes an option of a much longer brake magnet wire, which can be spliced, soldered and shrink tube wrapped and pushed up into the belly pan, virtually eliminating the issue for the life of the magnets. Luke, in Ft. Collins CO, who did my axel orders that option for all the ones he installs.
Another test you can do for your brakes is to take a simple ammeter and use it between the pins on your trailer cord. Find the brake wire and the + 12 volt
wire and measure the current draw of the brakes. Each magnet should take about 2.5 to 3 amps if they are all hooked up and working well.
Of course, working magnets does not always equal working brakes, but it is a good place to start.