The 7 Round Pin Tractor Trailer Circuit Tester from Innovative Products of America is designed to quickly test tractor and trailer lighting circuits. Just plug it into the pig lead coming off the back of the tractor and you can even take the tester in the cab and watch the LED's to confirm brake, running lights etc. Also the 7865 Circuit Tester can allow the driver to remove a pin in the tester, thereby bypassing a bad circuit, or putting a jumper from the center pin to any circuit to power it up in emergencies!