I want to put a second battery (for my radios) in my 1994 F-150 pickup
truck and connect it to the original battery (for the starter and
engine) with a relay such that the batteries are in parallel when the
engine is running (and the generator is presumably generating) and are
separate when the engine is not running. I did this with my previous
'84 F-100 pickup, but I can't find the "on when engine is running" wire
in my F-150. Does this wire appear anywhere under the hood?

--W0PBV.