This article is from the Car Audio FAQ, by Ian D. Bjorhovde (firstname.lastname@example.org) with numerous contributions by others.
The remote turn-on lead that most head units will not provide very much
current (usually 250-300mA), so there is a limit to the number of
components you can activate with it. Generally, it is safe to hook up
two devices to the lead without having to worry about problems.
However, if you'll be activating more components, then you should
probably use a relay.