87 Fleetwood Brake Light Problem

I have an 87 Fleetwood with brake light problems. The center (high) brake light works, but the left and right brake lights don't. Also the right rear hazard light does not work even though all of the bulbs are good. All turn signal lights work(front & rear), taillights, parking light, and side marker lights all work fine. I understand there may be some sort of relay that goes to the lower brake lights. If there is, where is it located and how do you check it.  Thank you in advance for any help that may come.