Originally Posted by Franks68coupe
Check your grounds on the back of the cluster, clean all connections etc. Look up a wiring diagram for your particular year, and try and get everything up to those standards (probably wired just fine, but good to be able to reference). Check your voltages to your individual lights...could also have your ICVR on it's way out back there. If nothing seems wrong there, the TS switch may need a looking at.
I've been using this diagram.
The left turn signal wire is #50, odd that just 1 cluster lamp flashes with the turn signal, when they are all connected to a common wire (#19).
19A is probably the problem bulb.
Looks like the consensus is a bad ground.
I guess I'll know more when I take it apart.