[AZKen]

1. You have purchased a gauge and a sender/switch, right?
2. Do you have the other sender that was for the light?
3. You are asking where to install "it". Are you talking about the gauge or sender?

Senders for lights and gauges are different, so you will need to use both senders.
You can mount the gauge anywhere you want. On the cluster bezel or on a hang down bracket or whatever.
Senders are mounted where they will read water temp. On a tee or extra spot if you use two.