The meter on controller prong of tow vehicle plug should be set to measure not the resistance, but the voltage.
Assuming this is cheap timing controller... When you press the brake pedal or controller "panic button" (manual override), the meter should read about 10-12V with motor off, and 13-14V in with motor running. If it doesn't, then there is a problem on tow vehicle end. Could be controller, or wires, or fuse. If it does, then there is a problem in trailer.
Attaching 12V lamp to controller prong is a better way, because you can see it from driver seat, and you can also see the delay in the lamp response, and play with the braking power slider and see if the lamp brightens and darkens accordingly.
Now, I don't remember which one is the controller prong, the one at "6 o'clock" maybe. Look in the vehicle manual or google it up, the 7-pin plug wiring is the same in most vehicles. If this all is too complicated, then you simply have to call a mobile mechanic that will come to your place, fix the problem in an hour, and charge you a few hundred bucks.