I think you've lost a ground (or two), and are back feeding through the other bulb filiments. And from your description, I suspect it's in the right rear unit.

Remove the bulbs from the sockets and check the continuity to the chassis. I suspect you're going to find some open circuits. Frequently, it's simple corrosion where the wire is screw mounted to the chassis.

'97 T-100 SR5
'86 Toyota's, the variety pack (all gone)