First I do not have a whirlwind but i do own a system 11. If im not mistaken those 4 yellow white wires are for your GI. I would put a connector there myself helps with troubleshooting. First thing to check would be fuses. Find the fuse that is feeding the string that is dim. Pull the fuse and check it with a multimeter. If it tests fine put it back in turn on machine and test for voltage on either side of the fuse. If you get voltage on one side but not the other its a partially bad fuse that needs to be replaced. If there is good voltage on both sides fuse is good. No voltage either side you have a break in the connection back to the transformer (solder joints in pic most likely). Assuming the fuse tests good you have to start tracing the line back to find where the voltage drops. The system 11 I repaired had 2 lighting issues that were from bad pins in connectors and had 1 shorting issue that was constantly popping fuses. Hopefully you get it figured out soon
Edit* didnt see the 2.4v part. Ya trace the wire that is feeding string back to transformer and see what voltage you are getting there. good chance 1 of those 4 wires has a cold solder joint and its not passing proper voltage.