Brake lights work on trailer but not on van.

hello goodguy, Thank you for the attention to my question. As I mentioned, all four taillights do work. I understand the upper taillights are also the brake lights - and have inspected the contacts. Is this testing for ground something you could describe how to do - I do have a continuity tester but never used it. (Guessing I need to touch one probe to somewhere on the contacts, and other probe ??) : ) BOTH brake lights do not work, does this help to narrow down where to look (i.e. I read somewhere about disconnecting the left taillight plug from the tail/brake/park assembly) but do not quite understand. Thank you. michael (PS - I should have said ALL FIVE BRAKE LIGHTS do not work. I did not tear into the set of three on the hatch.)

to check for grnds you need to hook up u r meter up to a good 12v battery source ( red connector) and then u just use the black probe to check for grds if it has a good grd the meter should read source voltage (12v) if it reads 0 then u have a bad grd