Really sorry to hear this and for your information, the Defi's come with this facility built in. Not only does it have a warning light built into the gauge, it has a high pitched whistling "buzzer" to notify you. However, I doubt you would hear this on a rally stage (as it is just audible on a track day), but the extra thing it has is the ability to connect a remote warning light as well, which can be mounted anywhere you like.

http://www.defi-shop.com/product/op/op_ind2.html

It has two different coloured LEDs, so one could be oil temp or pressure and the other water temp .