First try putting on the 4-way flashers, - if all the turn signals work, you know all these circuits work! Now some of these have separate TS and brakelight circuits, - some run through the TS switch! So of course you have to see which you have!
I think it is not hard to get to inside of trunk and get to lights, -- you can pull bulb an look at it, - and it is possible that it has same kind of bulb in another socket! If so you can swap and see if they work the same as before, or problem now switches- if it switches it is the bulb!
Now there is a switch down under the dash that brake pedal arm hits on when you do not have your foot on pedal, if this switch is broken, stuck, or unplugged the brake lights will not work!
If these things all check good, then you have socket grounding problems possibly, - and if they use same bulb for brake and ts both, then possibly the blinker switch is bad!
By the way the bulbs are not that expensive, - buy one and try swap outs, - if that is not problem, - it is nice to have a spare bulb in glovebox anyway! I always carry one headlight bulb, and one of each of the outside bulbs, - since I figure it is better to change bulb than being in an accident!