Auxilliary Battery Locations
The Y wire ties into the auxilliary lighting circuit (Y and Bk/Y) at a heavy splice inside the dash harness, which connects directly to the battery through a Br fusible link at the starter relay.
The continuous-duty relay (metal case) mounts at the 2 factory dimples in the firewall, and is triggered by a Wh/Pu wire that taps off the back of the fuse block & passes through its own rubber grommet in the firewall.
The aux battery hot wire (R) runs inside the L fender to the optional aux. battery tray shown here:
This option was not available after '86 because of the EFI air filter's location.