My knowledge of VW cooling systems is far from comprehensive; however I am unaware of a header tank which has an integral alarm. There are tanks which have contacts to monitor the current flowing through the coolant & any significant change would alert the external electronics; it is one of these which I have fitted. This was prompted by a cap & two original bottles failing in quick succession & for the price of an Ebayed VW tank, which had the reputation of being reliable, I considered it worth a punt .The pipework is compatible & easily adapted if one is adept at origami, although an additional bracket is required to fit it to the car. I suspect that it won’t blow off at the same pressure point as the original, I’ve been unable to track down that information,(although I didn’t really put my back into it) fortunately that point hasn’t been reached & thus further comment can’t be made.
I designed & built the original alarm circuit myself, which was fine & surprisingly immune to spurious slop signals, however a used Engine Watchdog TM4 turned up for which I gave £18-00. This particular Antipodean unit, which has now been superseded, monitors engine temperature, alarms on low oil pressure on top of the low coolant alarm. This would be highly recommended if the price were close to that which I paid, even three times the price, but as has been said it has been replaced in the range & when available was 150 Australian dollars. This is still cheap money for the savings it could make, especially fitted to a Ute thumping through the outback with mercury popping the thermometer bulb out at 120degree f.
I now await a pithy comment from Cobber & possibly Minimax. M