That's why factory "idiot" lights are not such a bad idea. They do grab your attention when they illuminate. You can (should) do something similar with lots of aftermarket options.
FYI -- There is a new gauge pod on the market that mounts a trio of gauges in place of the single radio speaker up on the dash for the 65/66 crowd. It not only maintains the defroster ducts but I think improves them. To Dave's point, it puts 'em where you will see 'em. I tried to find a link to post but no luck yet. I thought it was made by MP Products but it's not on their website. I saw it on E-Bay a couple weeks ago. I'm not familiar with the dash on later models or if this would even be available for them...just putting it out there for my fellow 65/66'ers.
Here is a link to the new dash gauge pod for the 65/66 crowd.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=330404152862&viewitem=