My passenger outer tail light bulb illuminates normally when the headlights are on, but doesn't illuminate the 2nd filament when brakes are applied.

I have tried swapping with the good bulb from driver side and same result. 3rd brake light works.

Anyone know how/where that bulb/circuit is grounded? Anything else to check? It's an aftermarket tail light housing but has been working fine for years.

I'd give the passenger side harness a try on that side.

That'll rule out whether it's that harness or wiring on the car side.

There is also a fuse in the panel under the steering wheel. Check there as well.

There is only one fuse for all the brake light bulbs.
On each light bulb, both filaments (brake and tail) use the same ground.

Remove the light bulb and check for 12 volts on the contact in the socket.

I swapped the bulbs again, filed the contacts within the socket and harness, and now it's all working.