Grounds are indeed good.

But the only way you can really know where the problem is, will be to track the voltage.

Get a good multi meter, and a 20' length of wire. 16 AWG stranded would be my favorite, but it doesn't matter.

Put one end securely to the negative battery cable, and the other to your negative lead on the multimeter. I suggest alligator clips.

Now, using a schematic, trace the voltage, from brake light switch all the way to the bulb.

There are doo dads specifically made to pierce the insulation, makes it easier. Or a fine sewing needle works too.

Good luck