Cleo Frank wrote:
[...]bought [a] battery[...] I did accidentally install it backwards

As has been said: high probability electronic items are smoked.

[...]the battery is now securely connected.
[...]no power whatever, clock dead, nothing.

I start with the lights, then try lights+horn.

A voltmeter is a pretty lousy tool for troubleshooting this
(well, after an initial check of the battery).
The light-bulb probes they sell in auto parts departments
are better for this--or you can make your own with any auto bulb.

Unlike a voltmeter, because these PULL SOME CURRENT,
they are really good at sorting out a YES from a NO from a MAYBE.
You can also tell Pass/Fail out of the corner of your eye.

Clip the lead hanging out the end to a part of the body
and start probing with the point.
When you stop seeing it light up, back up.
If you can't get anything from the body to the + post of the battery,
the ground connection from the battery is open.