Good thorough description.

This is one of those logic problems that won't be solved by referencing the Mitchell drawings, because they do not detail the paths taken on the instrument panel circuit board.

What is happening is the two 3W halogen cluster illumination bulbs are finding a path back to ground through the directional indicator lamps, as is your tachometer. The problem originally occurred when someone had the cluster out of the dash without disconnecting the battery. The clock wiring shorted momentarily and caused a foil trace in the ground path to open on the cluster circuit board. The cluster needs to be disassembled and the trace mended.

This can be a zero-dollar fix, and a good introduction to using a soldering iron.

Look for MTD240 Michael to weigh in on this as it is very much in his recent memory.

--
Art Benstein near Baltimore

Down in the depths of our souls, we all yearn for a job where rubber stamps are used.