At 07:45 PM 2/9/2008, Mike wrote...
>Every aftermarket radio I've ever installed had 2 power leads; one
>smaller lead to battery + to keep the station presets and the clock
>time, and one for accessory switched power + for the main radio's
>power.
>
>I've never seen one with a 'signal' input to trigger it on and off.

That's very strange, you must not have read my post before jumping in,
since I very specifically mentioned the accessory lead. Don't think
that the term "signal" is exclusive to an audio signal. That is exactly
what the accessory lead is, it serves to signal the head unit to turn
on with the ignition. It draws next to no current. Actual operating
power is provided through the battery lead.

If you look at the fusing on a head unit, you'll find that the larger
fuse is associated with the battery wire, and a smaller fuse on the
accessory wire. The accessory wires are often not fused at all, because
it's simply not possible for them to draw enough current for there to
be a problem. There's often a noise filter in the battery lead, too,
because that's where the actual operating power is drawn.

For example, here's a Clarion installation manual. There's a 15 amp
fuse in the battery lead, and none at all in the accessory lead.
http://clarion.com/us/en/MungoBlobs/777/794/DXZ375MP-INST.pdf