The fuel gauge in my 66 Coronet read only between 1/4 and 3/4 for a long time. Now, it has crashed completely. It reads less than zero all the time. Is it likely the sending unit finally crapped the bed, or more likely a wiring issue? Thanks for advice.

Easy way to check is to stick your test light into the sender wire and see if it blinks. If it does then you know the wire to the gauge is good. Check the gorund strap that bridges the rubber line at the sender. If it's there then you pretty much can call the sending unit bad. Could be the float sunk from leaking and filling with gas. The only way to know for sure is pull the unit and see what you got.

Sounds like the float has filled up with gas to me.
Interstingly.........
I recently replaced the 5/16 sender off my superbee with a 1/2 one from year one.
Never had any problems with the old one but the new one only reads 1/2 full when the tank is full

Read this before condemning anything:

http://www.moparfins.com/Repairs/Fuel_Gauge_Repair/Fuel_Gauge_Repair.htm

It's a lot easier to start at the sender end of the system then having to tear the dash apart to test.