Well, I found the (surprisingly simple) procedure detailed in the most unlikely place. Wait for it... the manual! :/
Unfortunately, I still can't figure out what bulb I need. In the picture of the rear of the car, it is simply labeled "taillights" (#2). Would that make it #4 on the bulb table, P21/5W? The weird thing is that the one that is burnt out is on the right side, but in the picture of the left side, the same light is labeled both 2 and 5 (rear fog light) and rear fog light is listed as #3 on the bulb table, P21W. It doesn't make sense that the left and right sides would be different, right? So which bulb should I buy, P21W or P21/5W?