"Someone way smarter than me must know the answer!"==== for your lights----That'd be Coolerman, trust me
"When the brake pedal is pushed the turn signal lights in the cab light up"
Not Normal Sounds like the brake lights might be seeking an alternate path to ground through the TS warn lamps--that'd mean a high resistance ground at the combi
Sure sounds like bad grounding and there are a lot of reasons for high resistance in the circuit besides rust--you had the right idea with the flasher jumper, you can try jumpering the chassis of each light to ground
disco and clean power connections
you can try jumpering power to each light and make sure it works
The combi light ground seems good enough for the brake light which does not need to pull current for a flasher to work but the flashing lights must pull enough current to operate the flasher---if the CCOT flasher is not the electronic kind and works like the OEM which I suspect it does from your problem
A couple of other weird things I have run into is good fuses that just needed cleaning (and the fuse holder) and also under the dash left side follow the warning lamp wire for the TS to the double female bullet connector and take it apart and clean the terms
at the bullet the front, rear, ts warn lamp, and power from the ts switch all come together so it is a good place to start
if you had a multimeter you could just ohm out the wires ---I bet one would show high but it can all be done with a couple pieces of wire jumpering power and grounds
I would disco the bullet connector and run a piece of wire from a fuse or unplug the dome light connector and jumper power from there to each of the TS bullet connectors if they light up then go make good grounds for the combis---this confirms they are not pulling enough current for the flasher or the flasher is bad (not likely being new)
mine is finicky enough to not flash with the eng off but will flash with it running 12v vs 14 volts. I know this means it is time for me to go through and do a good clean up on all the terminals
Just cleaning everything up make cure the prob and while you are at it put some di-electric grease in there and keep it from happening again
A real common problem that once you've been through it will never stump you again---we often look for an answer more complicated than the prob, human nature I guess
let us know how you get on with it
