The wear light has two functional modes:
1. When the wire loop is earthed, warning light comes on. This is intended to occur when the pad contacts the disc and the pad has worn down to the wire loop.
2. When the wire loop is worn through (becomes open-circuit), the warning light comes on. This is intended to occur when the pad has worn down a long way.
Now on my previous 164, I had the same symptoms that you're having and it was because the pad warning light wires (two of) were chafed on the caliper/brake disc, etc. If the wire becomes 'earthed', then the light stays on. If the wire gets chafed right through, then the light stays on...
For some of the more recent Italian models (probaby not the 164), the pad 'loop' is replaced by a single wire. At the plug connector, the plug has a wire loop so that the circuit is always complete. Then, the light comes on only when the wire is earthed. Still the same problem of course - if the insulation gets chafed through, the light comes on.
The solution is to get some 'spaghetti tubing' (woven fibreglass insulation sold in electrical shops, as used in ovens/heaters). Cut the plug off (or remove terminals etc.) and thread the insulation over the wires. A bit of insulation tape WON'T do...
I'm not saying the other suggestions in this thread are not valid, just reporting on what caused the problem on my 164
By the way I think my 164 had the ABS-style connectors inside the wheelarch for the pad wear wires, but this varies from year to year. The real ABS connectors are on top of the suspension struts, in the engine bay.
-Alex