There's only one light bulb in the cluster, so if it flashes with the left, but not the right turn signals, it's not a bad ground or bulb. If the exterior turn signals all work, I'd say it's a bad wire from the switch to the cluster bulb (there's 2 wires running to the same bulb ... ground is through the instrument cluster ground.
The inside belt moldings on the rear quarter trim are held on with staples. When you get a new belt line kit, it will come with new staples and a carbide drill bit for drilling new holes. The belt line kit will already have holes in it, so put it on, use those holes as pilot holes and drill new holes in the interior quarter panel with the supplied drill bit, insert the staples, and bend them over with some channel locks. They're a real PIA.
The inside belt moldings on the rear quarter trim are held on with staples. When you get a new belt line kit, it will come with new staples and a carbide drill bit for drilling new holes. The belt line kit will already have holes in it, so put it on, use those holes as pilot holes and drill new holes in the interior quarter panel with the supplied drill bit, insert the staples, and bend them over with some channel locks. They're a real PIA.