rear lights flashing with the indicators normally due to bad earth.
I would check by running a single (no joints) wire all the way from the light cluster back to the battery, can be done quickly round the outside as you are checking.
Fluctuating voltage could be because you are not measuring to a good earth.
Electrical oddball faults (other than blown fuse) I always start by checking the earths resistance back to the battery, I have a long negative lead for my meter to do this.