Its the alternator output wire. if it goes through a grommet you have a car which came with an optional 60 amp alternator and probably a the rear window defroster.
If it goes to the bulkhead connector you have a standard setup. On the diagram its wire R6.
Hm. That might be a home made version. But yes there is one blue wire coming from the bulkhead. That's the ignition run wire from the key switch. J2 or J2 something and originally had a white stripe.
When the key is in run, that wire gets power.
The power then splits at the junction. So each wire there goes to one of the items discussed above.
Cudamark recognized the first item photod
From the T, 1 wire continues onto the ballast resistor