Hi Karen d,

    Your question can only be answered with an "if". Any electrical system
or outlet on an automobile has available the battery amperage, or the output
amperage of the alternator. For example, if your battery has a CCA of 500
amps (cold cranking amperage), then you have 500 amps available to run any
consumer. This is in theory only, that does not mean you can get away with
for long periods. A starter run with a 500 amp load on it would throw the
lead out of the armature and soon burn up.
    Now comes the "if". The circuit is limited by the amount of amperage it
will carry. This is controlled by several things, the fuse in the circuit,
the size of the wire, how heavy the lighter connection is, how good of a
ground it has, etc. A good example would be the starter, it has a #1 gauge
feeding it straight from the battery. This lead (wire) is capable of
carrying the full load of the battery to start the car, and is grounded
directly to the engine/frame. This load is maybe 75 to 100 amps load, and
its for very short runs of 30 seconds at a time.
    But your lighter plug is feed by a #14 gauge lead. This size wire will
only safely carry 10 or 15 amps, and likely has an 8 amp fuse.
    You will need to know ahead of time how much load your consumer will
draw. And if your new circuit will be able tell to carry the load. I would
not try to go over about 10 to 15 amps draw for short periods. Being sure to
check to see if the circuit is heating up at any point. Bad grounds, and
loose connections will heat up first.
    Radio/Shack has a general automotive electrical manual showing load
carrying rules for wires, connections, etc.. You will still need to know how
much amperage your device will draw, but the book is a great help getting
started.

I hope this helps.

Howard