Brake lights

Have you checked the loom connections and the lightbulb on the non working side?

turn signal switch

If one of the brake lights is working, it's not the switch. I agree that it could be the turn signal switch, but only after verifying that you don't have a wiring/ground problem.

Remove the bulb on the side that is not working and clean up the socket and the lead pins in the pigtail. Check to see if the bulb is good as well or just replace the bulb with a new one and see if that works. Also check the grounds for the light. Can't remember if there is a ground wire in those units or not but if there is not then the ground is through the taillight bucketets. If this is the case you will have to remove the nuts that hold the bucket to the quarter and clean the areas between the studs and the fender in order to get good ground.If there are ground wires on them, then remove the end that is to ground and clean under the screw/nut that attaches it to the ground area.
Don

All of my bulbs are good. All the wiring is good as far as I can tell with my eyes.

regarding the turnsignals, I wonder if they won't work because I don't have any bulbs in the front side markers. At the moment, I can't put any bulbs in there because they are full of dirt.

Thanks guys!

Turn signals will work without lights in the side markers. You've probably got a bad ground at the mal-functioning tail light bucket or a bad turn signal switch (brake light circuits go through the turn signal switch).