There's always been something about the standard tail lights that has been a mystery of sorts to me. On my 69 Z the outboard and inboard bulbs are on when the switch is in the running and/or headlight position. If I use the bulbs that it indicates to use in the owners manual the inboard bulb is always brighter than the outboard bulb when the light switch is on, and the state of Delaware doesn't like that. I had to find another lower candle power bulb that matches the outboard 1157 bulb in the running light position(not sure what number I used). I always though that both the inboard and outboard lights were supposed to flash when using the turn signal or apply the brakes, but they don't. The inboard light is just an ornamental light (that's what I call it anyway) and doesn't have any other function but to come on when the light switch is on. As for RS lights it may be different., I've not had the priledge of being near one to see how they work. I'm curious what others will say.
Look at the rear harness and lamp sockets - the inboard lamps only have two wires - power and ground, so all they do is light a single filament all the time, at regular taillight brightness. The brake lights and turn signals only operate the outboard lamps, which have three wires (low running light filament, high brake/turn signal filament, and ground).
My original bulbs are double-contact #1157 for the outboards, and #97 single-contact for the inboards, and all four are identical brightness in the running light position. The owner's manual doesn't say anything about the inboard bulbs - it just mentions the tail/stop/turn lights, which are only the outboards.