[dsraven]

sorry, maybe I read/understood this wrong.
if you are looking to connect something to the ign hot terminal and you want it to work only when the key is on in the engine run position, then connect it to the ign terminal. use a fused relay if it is anything that has a decent draw because the ign switch battery supply wire isn't huge to run a bunch of power hogging accy's.
if you want to have that accy work but don't want the engine running then connect it to the accy terminal (again, the relay disclaimer).
if you have something that you may want to run for extended periods then you may wanna think about not connecting to the ign terminal because the ign coil can get really hot or if you have points ign then the points will likely fry quickly.
some guys will run a switch by itself, connected to a seperate freshly wired (fused at the battery) power source, so they can run their accy any time and not worry about overloading an old wiring harness. a relay is also an idea if you want a small switch that can be hidden out of the way. the little switch makes the relay turn on to supply power to whatever. small gage wire from the switch to the relay, larger gage wire and a fuse from the relay to the accy.
so, at the battery there is a fuse of sufficient size to run the accy plus a margin for voltage drop due to the length of the wire run, then the proper gage of wire to the relay, then more of that proper gage of wire from the relay to the accy and finally the accy has it's own ground to complete the circuit.
see the attachments for some ideas for fuses (auto reset circuit breakers) and a wiring draw chart for wire gage requirements used for the amp draw and run length.
answered?