As others have said there's much more than just the magnets. If you have a welder I have a trick I do to every trailer I own. I drill a 1/4" hole in the frame, stick a stainless steel bolt through it, and then weld the bolt head to the frame. I do it next to every light, brake, and one by the hitch. As long as you use stainless steel nuts/ washers you'll never have a grounding problem again. That being said you still have to have good wires connecting to the brake magnet and/ or light sockets.
Brake magnets always come with a short wire that people love to crimp a splice onto. Sometimes they will put some electrical tape over the splice, other times nothing. All it takes is a little salt or something else to get on those wires and suddenly they don't work correctly. A multimeter may not pick up on it as it uses a very small amount of power to test the circuit where as the magnet will draw a lot more power. It's more work up front but if you do the wiring correctly you will most likely never have a problem afterwards.
Here's my rules of thumb. Any copper wire that's not a nice shinny copper color should be replaced. All joints should be soldered and sealed heat shrink tubing to cover up the joint. All grounds need to have excellent contact to the steel frame that will not rust or corrode down the road (hence my stainless steel bolt). If you can't do that then run ground wires all the way back to the hitch. For trailer brakes don't connect all 4 (or 6) brakes in serial (serial is when you connect one wire to the power then the next wire connects to the wire you connected to the power, then connect the next wire to that wire and so on. If the first connection fails then you loose all brakes. No wire should be loose, hanging down excessively, or anything else that invites damage. All bulbs/ connectors to sealed lights should have dielectric grease on them.
