This a good question. When I was reading the instructions for installing aftermarket Sunpro gauges, they said you need a tee, which can be bought at an auto parts store. You can buy a pressure switch to fit the tee. You can have a warning light and the original pressure gauge. You can checkout Sunpro's website: www.sunpro.com The installation instructions can be found in a PDF file.

What size tee do you need? I'd like to know the answer to that question.