Yoshismom, First thing is you will have to put down your tailgate, where your rear window goes down, in order to get at the bolts that are holding your brake and tail lights in place. And as mentioned by silversup get a test light to test your brake and park lights. It may be a bad wire, a broken wire or even the same problem with the light bulbs. But you will have to get at them to check them and if you have a 3/8 or 1/4 socket set it will make it alot easier to get those bolts off, that are holding the lens on for your brake lights. They would almost certainly be rusted a bit so you may also need some penetrating oil WD40 or something like it but take your time , soak the bolts good with penetrating oil and they should come off, then you can test the wires and bulbs. The Bolts for that lens are on the inside of your rear section where the tailgate closes next to the body of your vehicle. Just lower your window, then lower your tailgate, and look towards the lens where your brake lights are and you will see 2 bolts holding the lens on. You also have to swing the lens a bit and work it until you have the lens come off the 4Runner and in your hands. There are 2 Studs that The Lens Is Resting On way inside and it may make it a little tricky to get your lens off but take your time and the lens will come off. Then you can check everything. Or have someone check them for you or take it apart for you.
Good Luck Yoshismom
OzzyO