Simultaneously.

Tracing Haynes chassis wiring diagram shows the only common link is fuse#23
15A. Where is this fuse located? what could have caused the circuit to
break? Radio installation?

pinoy wrote:
Fuse location? under-dash fuse box?

As for why it broke, a radio installation is a very good guess. Working
around inside a tight dash it is very easy to short things to ground.
There is a dash lights wire going to the factory stereo. Most
after-market stereos have an input for this as well.

If the installation was done by cutting the factory wires (ugh) and
splicing them to the new stereo harness, it would be very easy to cause
a short. It is also possible that the dash lights wire was left
unconnected and allowed to dangle around in there just waiting for the
opportunity to short to ground.