Annoyed with Toyota for all the widgets that my '91 had that they could not manage to install on the much more expensive 2013, I put one in my clock panel. I found a little .36” voltmeter module on Buyincoins.com for $2.94. Item (P011851). My detailed install doc is too big to attach, so I'll summarize with a few pics.
Pull stereo panel, pop clock panel out of back.
Pull hazard switch out of front (pops out)
Pull rear cover screw, release tabs and pop out rear PCB
Unscrew clock to get it out of the way
Carefully pry off front panel with a putty knife. (Held with double stick tape)
Cut opening to fit voltmeter module behind silly car icon
Mask and strip black paint from rear of front panel to make clear opening for the display. I used brake cleaner and electrical tape.
Pick up power and ground from existing pins on PCB and solder voltmeter leads.
Reassemble. Very clean, no new wires or connections.