Background on what I'm trying to do:
This question appears somewhat similar in concept to Can I control a bathroom extractor fan independently from the light? however, in my case, I believe the wiring was originally separate and was later retrofitted to be controlled together, so running new wires and installing double switch
A bathroom has a triple switch plate with two switches that are redundant...if either one or both are on (pointed up), BOTH the mirror lighting and the ventilation fan run. (The third switch controls a ceiling light). We'd like to control the fan and mirror lighting independently so we can install a dimmer for the lighting as well as have the option to run the fan without the lights during the daytime.
Although installing a dimmer switch in the other bathroom where the switches had not been paired together was straightforward, un-pairing the fan and mirror lighting switches correctly before installing a dimmer seems essential to prevent damaging the fan.
The real question:
How would the wiring for an existing bank of switches be modified to pair two of the switches? I'm hoping to see a wiring diagram for what a typical setup for a set of switches like this would be before coupling, and a diagram showing how the wiring is modified to couple the switches.
Edit: here's a photo of the switches box:
and because it's really hard to see what's going on in there, here's a totally unprofessional diagram of the wires that are easy to trace/look relevant. The little boxes in the center of the switches on my diagram are the holes on the back of the switches, and the little boxes on the side of the switches are the screws. The yellow things are wirenuts and each has a ton of wires coming in and out and its hard to follow what goes where since everything is stuck together with paint. but it looks to me that the wire indicated in orange should go to the hole on the back of the middle switch and the wire indicated in red should be removed?
edit2: after loosening the screw terminal on the left switch it appears the orange and red wires in my diagram are actually a single wire that had an inch of the insulation stripped to wrap around the screw terminal. Would electrical tape adequate for replacing the missing insulation? Or does it need heat-shrink tubing or to be cut and covered with a wire-nut?