If I was in the US I would build you a voltage regulator and mail it. But any electronics hobbiest would be able to do it for you. I've done it a few times for a family member's Chev V8 truck (he damaged the first one, and when he sold the car I built him a new one plus spare). It is about $10 or less in off-the-shelve components. The big question is just at what voltage you want it. I recall 10V being correct, but the Chev was 5V. But that was much newer aftermarket gauges.

The voltage stabilizer (in general, I'm not refering specificly to the Volvo implementation) works as the first reply stated, execpt that after a while the switch closes again as the bimetal cools down. It opens and closes about every second or two or three or something. On average over a period of time you get 5V or 10V (depending on design) to the gauges, but this is switched 12V/0V with some duty cycle in order to get the designed average voltage. And the 12V is to give a quicker response. In fact, if you watch closely, you will see the gauge moving fast, then slow, then fast, then slow. I can especially see this on my 240.

A regulator would always supply the designed voltage, a stabilizer gives on average the designed voltage. Since this thing switches between 12V and 0V it is called a stabilizer and not a regulator, but it does virtually the same job. Modern light weight high power power supplies like in your computer use a switching regulator, which is just an electronic version of the voltage stabilizer but switches at a much faster rate. About 100 000 times per second. The advantage is that you do not have a voltage drop across your regulator, and we all know that power (and thus heat) = voltage x current. No voltage drop means no heat generated means longer lasting.

Have fun...