Installing dual light/exhaust fan in bathroom

Hi-
I am getting ready to swap out the exhaust fan in our bathroom with one that has a light in it as well. The instructions for the fan show a 4 wire connection which we currently don't have in the bathroom.

Are my options to run a new 4 wire Romex in place of the current 3-wire or just run an additional 3-wire? There is a ton of blown-in insulation in the attach so this may be a pain finding what I need. Once I am in the attic and find where the wire goes down into the bathroom what exactly am I looking for? Do I just following it back up into the attic and look for where it is spliced into another line? At this point, I would just tie in a 3-wire cable and run it into the bathroom correct?

thanks.

 

Ground is not counted. You probably have 2-conductor cable (black, white). First step is to find out if power comes in at the fan or the switch. If you have a single 2-conductor cable at the switch power comes in at the fan.

3-conductor cable is black white, red.
4-conductor cable is black, white, red , blue.

 

Thanks. I need to get up in the attic and take a look. Here are a few pics of the fan switch which is on the left of the dual gang. The switch on the right is for the lights above the mirror. I haven’t had a chance to pull that one out but it looks like a rats nest in there.