[shorod]

The taillights are part of the same bulb as the brakelights be separate from the amber lights, correct? And it sounds like the amber lights work fine, right? Or are the amber lights you're referring to the front marker lights? It is possible that the "dim" filament of the taillight bulbs are bad and a few new bulbs will fix your issue. Before spending more time under the instrument cluster, I'd suggest you check for voltage to the low filament of the taillights when the headlights/parking lights are on.

-Rod