T-Tap Power Harness for Harley-Davidson® Motorcycles

Does this eliminate the need for a manual on/off toggle switch? Specifically, does it turn the amp on and off when you turn the bike on and off? (2007 road king)

Correct.  This T-Tap will give you the connection you need for a switched power source (a source that comes on and off with the bike). Just be sure your bike model and year are compatible.

Will this work on a 2013 Road Glide Ultra?

Yes, this will work on a Roadglide Ultra up to 2013 model year.

I have a 2005 deluxe would I need this for my bike if I do will it come with instructions on how to install

This is not needed but will make installation easier.  This will easily provide the ground and remote-on wiring.  Alternatively, these wires can be tapped in to and connected directly.

What’s the total length?

Total length of the H-D motorcycle T-Tap is is 6.5"

I have a '94 softail. What would I need to do to make the speaker kit work on my bike since this harness only goes back to '96?

This harness is optional and not needed for installation.  It makes install a little easier by providing the switched power wire through a harness instead of having to splice in to it.  Also, the ST600 system does not need this or to splice in as it uses a remote with an auto-off feature instead.